Is there any chance to install any custom firmware on RT-AC59U?

[xiltepin]

I am trying to connect to a vpn as a client but my provider is telling me the custom openvpn of my router is too old so I can't connect. Tried installing Merlin but no luck.

 

[RMerlin]

You need to find a firmware developed specifically for that model. Firmwares aren't generic, and I don't any for that particular model.