It seems like you are using the same mlwifi chip type as my Linksys WRT1900ACS https://openwrt.org/toh/linksys/wrt1900acs eg. Marvell 88W8864, which had significant firmware problems and issues with the 21.02.x release which should be fixed on the 21.02.2 release. Have a look at this thread:
You could try using the latest snapshot release candidate for your device so have a look at:
For the 5GHz Problem this could be for example that you use a Channel with DFS (Radar Detection) I think depending also on the country you are, but probably you have the same issues I had with my Linksys WRT1900ACS.
Also a look in the logs would be helpful, for example under GUI -> Status -> System/Kernel Logs.