Hi 5teve,
It’s been a while since I’ve used OpenVPN on the slate as I now have another server I use for Wireguard (still behind the RT-AC68u). If I get a chance today I’ll see if I can connect to OpenVPN. That being said, make sure you have “Client will use VPN to access” set to either “Internet only” or “Both”.

Also check your config files to see if they are set up to give Internet access. I know Merlin has made some big changes to OpenVPN in the past firmware or two so it may be a good idea to make new configs and re-connect the Slate.
Let me know if this works!