This guy uses express vpn because it gives him a block of five choices (with their vpn router option) one can easily choose on the fly for country.
HOwever its openvpn only…
Read on from this link to see how he accomplishes this…
http://forum.mikrotik.com/t/can-not-access-the-ip-range-for-the-out-interface-in-my-wireguard-tunnel/164953/39
The wireguard is a distraction as its just to get remote access into his network and has nothing to do with his setup.
What he does is have two standard ISPs, they are only there to provide access for his open VPN (wrt router) connection.
He then uses another port on this openwrt router to act a a third wan connection for the MT router.
All his MT users then use the WRT VPN connection for internet and then they can use the various 5 country options for where they want to get their internet connection.
Not sure if that is what you want or not… but sounds like it.