Ah, definitely see how things improve with the routers. If you have an USB powered device and HA is on an UPS or something so it doesn’t go down, you could try and find a USB battery bank to use as a UPS for the device, it would run for hours as it doesn’t pull much current - but some batteries turn off if the current draw is too low.
As for why a device won’t rejoin - you may want to open an issue at the radio library repo - zigpy-znp.