I am having some problems with one of my omnitiks. One of my problems is that working withi higher frequencies i get lower signal. Is this normal? 3 of my client cpe-s that are 5km distance away from the omnitik does not connect because of low signal when i use higher frequencies. Is it possible that the radio or antenna is damaged? Another thing that i have noticed is that i can achieve better throughtputs with nstreme rather than with nv2 but with nstreme one of my cpe-s constantly dissconnects and reasociates. So nstreme is not stable in my situation. But with nv2 i can get half of throughtput. How can i solve this?

This is probably according to the power scheme related to your regulation domain - country. If you have use it. Check your local rules that define allowed transmit power according to frequency used. 5km to client is quite long distance. What is on the other side?

I have set Frequency Mode to manual-txpower not to regulatory domain. on the other side those with the long distance are SEXTANT G 5HPnD. those far away have better throughput with nstreme protocol but when i change the protocol another cpe that is located close keeps disconnecting and associating.

There may be difference up to 1-3db from lower to higher channel. nstreme is not good for p2mp scenario with more clients per AP especially if u use OmniTik (nstreme disconnect problem is old story by the way)
When u do test with nv2 do it with more connections.

I would use sectors as ap. Specially for long distance clients. use different antenna for close and for far clients. It will help. Also, build your llinks according regulatory conditions.