Hi pc,
Did you look at your router's documentation?
https://github.com/RMerl/asuswrt-merlin.ng/wiki/Static-ip-for-OpenVPN-clients
BTW, yes, you
can run easyrsa on a router, but you definitely
should not! Embedded devices have inadequate sources of entropy, which is required for random number generation needed by
openssl(1). And also, it's insecure design to host your CA on the server machine; if the server was compromised, so is your entire PKI.
regards, rob0