Not able to enable 5G on my 13 pro max

Just to be sure, by "5G" are you referring to a wireless or cellular network? Regardless, these are not settings that you enable on your iPhone. They either exists in the vicinity of the phone or they don't.

If you are sure that they do, a few things you can try are:

• "Force" reset the phone. Ref: Force restart iPhone - Apple Support
• Reset the network settings on the phone. Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings