I should probably try what the O.P. did as I also have an ASUS router with Merlin. However, in my case, I have a Synology NAS and I set up VPN on that and use that server as my VPN. I have a certificate-based sign-in through a single port on the router.
I use a Public VPN service when I'm out and about and then connect to my VPN when I need to do stuff on my network, Like syncing my Joplin notes.