hi,
if possible, i would convert the dsl router into a modem, somethings called a straight-thru-bridge.
i have done that over 100+ times over the years.
if you cannot do that, a port forward should work, but you might need to make some changes to the openvpn config.
on the dsl router, how did you setup the port forwarding?
can you be specific, which ip address?
post a screenshot.