Is there any option, perhaps in firewall config page of AIS router, to allow vpn protocols through ? Such as L2TP, IPSEC, TLS, etc .
Another option might be to use a DMZ port on the AIS router if provided. This exposes the VPN router directly to the internet ( as does bridge mode) so you need to have the vpn router configured properly to prevent remote access and hacking. Hopefully, the vpn router is getting routine security updates ?