I've ran a Watchguard XTM-26 to an MX device for several years. The XTM is now likely a model T15. Several tech notes out there on how to set it up.
The WG devices come with a license when purchased but the device does not shut down when it expires, so you can keep using it. Makes a decent VPN endpoint, although I'd likely go Z3 with a long license assuming this is for a small office, partner company or individual.