DH key too small errors with RT-N66U

A few weeks ago I tried setting up an OpenVPN server on my RT-N66U in the states before going overseas, but when I try to connect to it I get "dh key too small" errors. I've googled as much as I can but I'm still unsure as to how to fix it... didn't think it'd be this complicated. I saw a recent post that suggested using openssl - do I need to download for the server? I see a lot of people using Merlin also, but I'm using just the newest ASUS firmware. Also, I don't have direct access to the router (being overseas now) - is it possible to fix this remotely?

Unfortunately I can't really make sense of everything yet - can anyone offer a step-by-step solution? Thanks.