Checkout this thread with many general best-practice tips on improving and extending signal range:
You want to connect more mains-powered Zigbee router devices first to make a stable Zigbee mesh.
Always-on mains-powered Zigbee routers act as signal repeaters and range extenders.
Suggest buying “IKEA Trådfri Signal Repeater” as dedicated routers that are less tempting to unplug.
Xiaomi devices are however infamous for not connecting to routers and only want to connect directly.
If using Xiaomi devices in a large home you need to also have mains-powered Xiaomi devices as well.
Best is to also have a better Zigbee coordinator like CC2652 with a great external antenna.
Combine CC2652, a great external antenna + mains-powered Xiaomi devices and you should be good.