Hi TrebleTA,
I'm afraid Full-cone NAT is not supported on DSL-AX82U, but we will consider your request to add it in the future firmware. Please also check your pm.
If you mean the same port twice to the same device in system log(one is for TCP, and one is for UDP), it is normal.
But if you have any issue when use Open NAT, please check your pm first.
For VPN client settings, please refer to the following link for the web interface to set up/activate your VPN client.
https://www.asus.com/support/FAQ/1011232/Thank you.