OpenVPN + Technicolor CGA4233-EU

[Azmodes]

Up until recently my network configuration was my ISP's modem/router in bridge mode and an old office PC with an additional network card running OPNSense on it. Worked mostly fine, but yesterday all of a sudden the WAN interface refused to acquire an IP from the upstream DHCP server. After a few hours of tinkering, I was at a total loss as to why (ISP said the connection was fine up until the modem, and indeed you could see the DHCP sending an IP, gateway, etc. in the OPNSense logs...) and since I've had strange connection drops every few weeks ever since switching to bridge mode, I decided to just revert to router mode, setting the OPNSense machine as a DMZ.

This fixed the problem, but now my VPN setup doesn't work anymore. I got OpenVPN running and it was working perfectly with the old config, but now the Technicolor being back to router mode seems to block the connection no matter what I do. As I said, I've basically switched off the inbuilt FW features of the Technicolor and set the OPNSense machine as a DMZ, so everything should get forwarded to it. Tried setting up specific port forwarding regardless, but it didn't make any difference. According to the client log, there's just no response. The public IP is correct, credentials are correct, I've even issued a new client certificate after the change, etc.

FW rule:       IPv4+6 UDP   *   *   WAN address   1194 (OpenVPN)   *   *      OpenVPN OpenVPN_Server wizard allow client access

Any ideas? Is this just something that the modem blocks by default? If you need any more details about my setup, please ask.

Thanks in advance.