Has this always been the case or just started happening @Miqi ?
Here are a few thread's with tips in.
Galaxy-s20-series/s20-fe-sim-manager-greyed-out.
Galaxy-note20-series/dual-sim-always-on-greyed-out.
Questions/dual-sim-options-greyed-out.
If all else fails such as a factory reset after backing up then let a Samsung Service Centre take a look.
I wish you all the best. 
