You can't force a device to use a specific Zigbee repeater. Zigbee devices choose what they think is the best route, but following the above advice is probably the best you can do (adding the repeater, waiting a day or two for the network to see that it is "good"--and I'd recommend checking the Zigbee logs mentioned above to make sure you don't see bad LQIs or RSSIs, and there are also some posts here that explain incost/outcost which I think are part of the route evaluation).
Is this the only repeater on your network? You could consider adding another of so (it's hard to know where dead spots are since we can't see 2.4 GHz with our eyes and can mostly guess based on distance and obstructions). Perhaps more importantly, you may want to make sure you don't have any "Xiaomi-unfriendly" repeaters on the same network. This includes most Zigbee repeaters, but the Ikea ones are all good as far as people have reported. Even if it manages to pair through the Ikea repeater (you can probably see this on the http://YourHubIP/hub/zigbee/getChildAndRouteInfo page right after pairing), there's no guarantee it will stay there. Zigbee devices constantly re-evaluate routes and may change based on what they think is better, though all of my Xiaomi devices seem quite hesitant to change routes and would rather just fall off the network (perhaps your experience as well).
I'm sure you've read this thread (or at least the first post), but if not, it's pretty much required reading if you use these non-standard devices on Hubitat: Xiaomi & Aqara Devices - Pairing & Keeping them connected
Good luck!
