As above reply, you first need to understand that your other Zigbee networks and devices connected to those have really nothing relevant to with with the ZHA Zigbee network, (nothing to do with other Zigbee gateways/bridges/hubs/controllers/centrals), with the exception that they might cause small interference with each other if using the same Zigbee channel.
Secondly you need to understand the roles that different device types play in each Zigbee network, (again, they play no role in other Zigbee networks than the Zigbee network that they are paired and connected to). If you have Zigbee Router devices (which are sometimes also refered to as Zigbee repeaters/extenders) on other Zigbee networks then they do not do anything on your ZHA Zigbee network, (other than possibly cause some small interference if on same Zigbee channel, i.e. using the same radio freqency).
So, you must add Zigbee Router devices to your Zigbee network in ZHA for them to improve your Zigbee network mesh there. Read → Zigbee networks: how to guide for avoiding interference + optimizing using Zigbee Router devices (repeaters/extenders) to get best possible range and coverage
Aslo read ZHA integration introduction and limitation below:
ZHA integration introduction as a Zigbee Gateway
ZHA integration is a hardware independent Zigbee gateway implementation that can replace most proprietary Zigbee gateways/bridges/hubs/controllers. It supports pairing/joining devices that use the Zigbee 3.0 standard and earlier Zigbee standards for home automation and lighting. It follows the Zigbee PRO specification, a low-power wireless communication protocol using small digital radios to create a private Zigbee network mesh that can connect compatible devices within a local area.
Before installing the ZHA integration in Home Assistant, you need to connect a Zigbee Coordinator radio adapter, that will act as the physical network bridge interface and single point of security controller for your Zigbee network. Those normally come in the form of a USB dongle that plugs directly into the same computer that is running your Home Assistant installation. The ZHA integration is compatible with many different “Zigbee Coordinator” adapters from various manufacturers. Be sure to note the recommendations in the respective sections below before buying a Zigbee Coordinator. A Zigbee network always needs to have one Zigbee Coordinator (it can never have more than one), and Zigbee devices can never be connected to more than a single Zigbee network, however, a Zigbee network can have multiple “Zigbee Router” devices and “Zigbee End Device” products.
Once ZHA has been set up with a Zigbee Coordinator it will automatically create a Zigbee network and you will be able to join/pair any Zigbee Router devices and Zigbee End Devices. With only a few limitations, most devices will join/pair directly regardless of brand and manufacturer. It is generally recommended to buy Zigbee 3.0 compliant devices if possible because those offer better interoperability, though a new Zigbee Coordinator is also backward compatible with older devices as well. Still, be aware that all functionality might not always be supported or exposed for every device out-of-the-box as some devices that use manufacturer-specific extensions to add non-standard functions and features could sometimes need device-specific code to fully work with ZHA.
Because a Zigbee relies on “mesh networking” technology it depends heavily on having Zigbee Router devices to expand the network coverage and extend its size. These are always mains-powered devices that route messages to other devices that are located close to them within the Zigbee network mesh to improve the range and increase the total amount of devices you can add. You should therefore make sure that you add many Zigbee Router devices and not just Zigbee End Devices or else its network mesh connection routes will be limited due to the short range and poor wall penetration of Zigbee radio signals. It is highly recommended that you read and follow all the general tips below about Zigbee interference avoidance and network range/coverage optimization).
Optionally you can configure Zigbee grouping of lights, switches, and fans (i.e. support for commanding multiple devices as a single group). In addition, you also have the option to bind some Zigbee devices, like binding a button on a remote control directly to a device or group (i.e. define relations between two or more Zigbee devices that will work even when the Zigbee Coordinator and ZHA integration is not available).
Limitations in ZHA integration
Note that ZHA only supports connecting a single dedicated Zigbee Coordinator radio adapter or module with a single Zigbee network and that the Zigbee Coordinator cannot already be connected or used by any other application. Any devices that are or have previously been connected to another Zigbee implementation will also need to first be reset to their factory default settings before they can be paired/joined to ZHA, please see each device manufacturer’s documentation.
Any Zigbee device can only be connected to a single Zigbee Coordinator (only one Zigbee gateway). This is a limitation in the current (as well as previous) Zigbee protocol specifications, governed by the CSA (Connectivity Standards Alliance). As such, it is a limit that applies to all Zigbee implementations, not just the ZHA implementation.
Support for commissioning Zigbee 3.0 devices via “Install Code” or “QR Code” via the ‘zha.permit’ service has so far only been implemented for ‘ezsp’ (Silicon Labs EmberZNet) or ‘znp’ (Texas Instruments) radio type in ZHA. Other radio types are missing support in their respective radio libraries for zigpy or manufacturer’s firmware commands/APIs.
ZHA does currently not support devices that can only use the ZGP (“Zigbee Green Power”) profile which is used in a few batteryless self-powered or energy harvesting devices, (such as for example; Philips Hue Click, Philips Hue Tap, and some “Friends of Hue” partnership switches).
ZHA does not currently support devices that can only use the ZSE (“Zigbee Smart Energy”) profile, that is however due to the “Zigbee SE” specification not being part of the standard Zigbee 3.0 specification and thus not implemented in most of the Zigbee protocol stacks that are commonly available Zigbee Coordinator radio adapters and modules.