Hi!
I am trying to find a solution to control my zigbee devices next door.
I cannot use my raspberry directly because it is to far away.
Is there any reliable fast zigbee controller (ZHA) you could recommend me?
I tried the old Sonoff Zigbee bridge with Tasmota, but it is a pain 
Thank You!!
You probably need to increase your zigbee mesh network by adding more zigbee router devices.
Your controller should be ok.
Most mains powered zigbee devices like bulbs or smart plugs also act as routers, so as @w35l3y says, just add one of those nearer ‘next door’ and the stuff on the other side might well then connect.