anything wrong with 5G connection of MR200?

Is the internet/WAN ip address on TL-MR3220 public ip address? Can you ping the ports which you want to open in the internal network? better to analyse your issue with the configuration screenshots...

Do you use SIM card internet access or ethernet access? Have you tried resetting the router?

Was it working properly? You mean no matter which external HDD you connect, they will disconnect and reconnect automatically? How often will this happen?

Better to email tp-link technical support directly for further help. They ususally reply email very soon.

If there is no any connection issue, better to set on "Auto" and the devices will choose the better channel automatically.