If your main router has two WAN (e.g. WAN1 and WAN2), it should be able to connect to multiple wan connections. But this may not be the case. So you should check your main router’s config to understand how it manage WAN1 and WAN2. Is it failover or load balancing?

So, as @elorimer said, are you connecting a circle? MT1300 should have a different Internet resource, right? Otherwise, you are connecting wrong.

If you want to access your local network, you should just connect MT1300’s WAN to your main router’s LAN. Then set up vpn on MT1300 and you can access MT1300 from vpn. You can set up port forward to access client behind MT1300.

But Astrorelay is the best solution for you to access your local network without vpn.