@AC_2300 

Hello, may I know which country are you from and what is the region version of the C2300 router, which can be verified on its bottom label?

After updating its firmware, did you reset the router to factory defaults and set it up from the beginning? If not yet, please give it a go first and check if it will help.

BTW, what kind of 5G devices are trying to connect to the 5G network? Kindly ensure the wireless adapter is also running the latest version.  

meanwhile, we can change the 5G wireless channel to different ones and try to find a more stable one.