What is the "correct" method to add a static ARP entry?

[ZPrime]

I think subject says most of it.

I have an ONT/ONU device on one of my WAN interfaces that has a management IP, but after a while it stops answering ARP requests for that IP (I suspect this might be a "security" feature but I'm not positive, but I don't think it's something I can easily change).

Easy fix is to just add a static ARP entry on my WAN interface, and an IP Alias to that interface in the same subnet, and I can have permanent management access to the device.

What is the best way to add a static ARP entry that will survive reboots as well as upgrades? There doesn't seem to be a way to do it in the GUI. I found this ~4 year old post that suggested adding some lines to /etc/rc.conf but I assume this won't survive an upgrade and isn't really the "right" way to go.

If it was on the LAN side, I know I could add a DHCPv4 static entry with the "ARP Table Static Entry" option enabled, but I don't know if this will work for something in a different subnet that isn't actually being served by the DHCPv4 daemon...