What I meant is that for MOST VPNs, one end must have access to a public IP at least at one end of the connection. The purpose of this is so that the peer SERVER can be reached by all users, be they from individual devices (laptop/smartphone) or from individuals behind another remote router. A static public IP is easy to reach, a dynamic one, means you need some dyndns type service to resolve the current WANIP address ( MT devices have one internally which can be used ).
So the question reframed is do either of the upstream Routers, that are connected to the internet get a public IP.
If yes, is either them under your control such that you could forward a port from the upstream router to the MT router.
In your case both upstream devices are ISP Routers…thus can you at least access the admin page or basic control page on the ISP router to forward a port, assuming the ISP router gets a public IP? Both MT routers will get private IPs as WANIPS, from the ISP Router LAN subnet.
Also would need FULL config of both MT devices. ( i see no firewall rules for example)
/export file=anynameyouwish ( minus router serial number, any public WANIP info, keys etc. )