There are links on the setup page that walk you step by step through this setting it up on iOS, Android, Windows, and Mac. Pretty straight forward with the most recent release.
Yes, you need to switch to using certificates.
Here's some more to read: https://github.com/RMerl/asuswrt-merlin/wiki#wiki-openvpn
I also recommend to enable TLS-auth for better security.
The key/cert to use for that is the simple static key in addition to the client/server certs.