What channel did you choose?Had simular issues with my wAP AC but noticed in the log after a while that it said radar detect and tried another channel and then loop. Added a channel that does not require radar scan here. This made 5 Ghz stabil for me.
Testes with all channels available. No result.Try setting 5G radio to a fixed channel. Don't know the implementation of MikroTik but it sure can help.