Hello! @busolof
Actually according to the log OpenVPN connected successfully and remained connected for several hours. Since Asus offered to replace the device, then something wrong that's specific to your own one might be the problem.
Even the fact that you say that you can't upgrade to Asus Merlin is unusual. In AsusWRT routers, upgrading to Merlin is a matter of a few clicks, literally. https://blog.usro.net/how-to-install-asus-wrt-merlin-router-firmware/
We're confident that the router replacement will solve any issue. Or maybe the AX56U has some problem that makes its behavior inconsistent with the AC56U and AC68U (which is an AsusWRT router we own and which we based our tests on). @giganerd
Reviewed the guide for AsusWRT and it is up to date.
Kind regards