No Modbus when WAN is down

Configuration
1

I need some help understanding my situation!

I started my home automation long before learning about Home Assistant. I have six web enabled devices scattered throughout the property controlling and monitoring much of my home automation. These devices https://controlbyweb.com/ provide digital and analog inputs, relays and one wire sensors. I monitor temperature and humidity, control dehumidification, open and close garage doors and list goes on. Last year I decided to incorporate this information into Home Assistant using Modbus to bring the information into HA. Everything always works just as expected without any problems.
Last night I experienced a power outage at my residence due to a storm. This is the first outage since integrating the Control By Web devices into HA. Just fyi, I live in a rural area, When my power goes out so does my internet. The cable distribution amplifier in my area loses power when my power goes out, NO internet. I have a backup generator that automatically comes online when utility power is lost. All of my network devices and computers are on a UPS so they do not see any interruption in power during the transition to generator power. My local LAN continues to function as you would expect. I can communicate with all of my network devices locally including my six Control By Web devices. My home network is up and running normally. Just no internet. When I opened HA I noticed right away that ALL of my 40 plus modbus entities were reporting unavailable! What’s going on here? So first thing I start pinging my devices. Everything is alive and well. I navigate to the devices local web interfaces and can see that all are working normally.

Can someone please explain to me why modbus integration in HA does not function properly when I loose my WAN connection? Seems pretty fishy to me since this should be local communication between the local device and my HA instance running locally on the same network. As soon as my internet came back online modbus began communicating normally again.

2

We need lots more information about your setup to be able to say anything.
It can be that you are depending on a service, like DNS lookups, which require internet and it can also be that your router behave strangely when no WAN is available, just to make two suggestions.

3

Tell me if I’m wrong. The modbus integration as I understand it is a locally hosted protocol installed on HA that is communicating with another local host on the local network. The connection between these devices would be stored in the ARP table in my switch/firewall appliance. DNS should not have to play any role in this communication unless the modbus Intergration has some sort of external dependencies that are not disclosed. The devices I’m communicating with have NO external dependencies. The data is served up from the built-in web server on the device. I could take one of these devices and connect directly to my pc and read the data generated if I wished. Looks like I may have to do some packet sniffing with Wireshark to see what is taking place.

4

It was just a suggestion based on a possible setup.
As I said we need a lot more information to be able to make a good guess on what is wrong here.

5

It will depend on how you have configured the modbus integration for each device. In particular what has been put in for host as it could be a DNS based name which would then require DNS to be running or an IP Address.