i am not sure but when i see on advanced setting interface vpn_client, is linked with tun0, may be tun0 is for wifi 5G and tun1 for wifi 2.4G ?
openvpn is worked only with one interface wifi and not both ?
i don’t setup anything, is a standard ar750 excepted i have modified default ip 192.168.8.1 because my network is 192.168.2.xx and i added custom dns 22.214.171.124 and 126.96.36.199. UPnP service is checked.
I use ipvansih provider for vpn.
I can use the VPN on both 2.4G and 5G network. Not sure about your problem. But if you want, you can send me the configuration for a check. otherwise, can you just reset the whole firmware and try again.
after editing gateway to ip ar750 all is working, myip from ar7500 is 192.168.2.33, on advanced setting the ip gateway was 192.168.2.25 that is routeur ip (srx5308 netgear), i have modified on adanced setting and i have replace it by 192.168.2.33. i use my ar750 as repeater to R6300v1 netgear wich is connectes to switch netgear, and this switch is conencted to srx5308.