Release ASUS ROG Rapture GT-AXE16000 Firmware version 3.0.0.6.102_36382 (2025/03/12)

[opq]

While connected to OpenVPN, use
ping -f -l 1500 internal_ip from a windows machine
ping -M do -s 1500 on Linux

Most Linux distros will return the maximum MTU, but ultimately by try&error you'll see what's the maximum MTU supported.
Also logs of the OpenVPN client are helpful in showing tunnel MTU size.

It's not an easy problem to nail down, but not impossible.

And don't try any fancy app. While connected to OpenVPN browser should let you connect to the internal IP of the router. If webUI load properly, that would be a good indication OpenVPN is working properly. Some old apps, like security cameras/DVR management apps are really bad at discovering MTU and there's no way to convince those otherwise - they send large IP packets, with LAN MTU and don't-fragment so they won't work.

Thank you. Unfortunately ping times out regardless. OpenVPN client shows data received and sent though and stays connected. Strangely everything worked on this AXE16000 until the 3.0.0.6 firmware update.