With 31 end devices connected directly to the hub, you can only have one router connected, because routers count towards that 32 device limit of the SmartThings hub.
As NoWon states, you should find a way to move some of those 31 Xiaomi devices off to be connected via a router instead of directly to the hub. That will allow you to use more routers.
EDIT: I have to apologize, because I was incorrect in saying that Zigbee routers count towards the 32 device limit. That limit is only for end devices as stated here.
I can also confirm that using the normally recommended “self-healing” method of powering down the ST hub will not work well where Xiaomi end-devices are concerned. They do not re-join after going past the end-device timeout which is the whole reason why people have troubles with Xiaomi devices remaining connected. It’s much better to do a “forced” (manual) re-join by putting the device into pairing mode (but don’t delete it from the ST hub’s device list, in it’s install location, to get it to connect through a router.
In your situation however, you will need to remove a number of your Xiaomi devices to make sure you have all routers paired and properly connected to the ST hub before attempting to re-join end devices to connect via the routers.
It’s a lot more work, but the best-case procedure would be to remove all devices, pair all routers to the ST hub first, and then pair all end-devices at their intended install location so they can choose their router.
Also, in my testing if a battery-powered Xiaomi end device appears as a router in the XCTU network scan, that device is connected via an incompatible router and will sooner or later drop its connection.