I would like an aerial (5 GHz, 28 db) install on a 80 metres high tower.
The Mikrotik server is in the cellar.
It is between aerial and Mikrotik server approx. 90 metres of cables.(CNT400)
This is my problem that cable is too long and the signal gets too weak.
How can I solve this?

Use a MikroTik RouterBoard in an outdoor chassis - mount it with the antenna. Then - you will have a short coax run of just about 1 or 2 meters, and the 90 meter ethernet cable run to the RouterBoard won't matter. Running 2 Ghz or 5 Ghz signals through 90 meters of coax cable will NOT work well.