OSPF over GRE tunnel

Hi!

Could You help me with routing and tunneling over LTE ntework?

I need to pass OSPF over a private LTE APN. On my side I have a miktorik router which terminates the GRE tunnel to a mobile provider and a static route to APN network over the tunnel interface. Then I connect a STX LTE to the APN and a branch office router (also Mikrotik) is connected to the ethernet interface of SXT. How can I connect branch office router to LAN OSPF router? I can’t route the branch office network to STX address because APN router doesn’t have any address in APN network. I’ve tried to EoIP from LAN interface of APN-Router R2 to LTE interface of SXT and have a static route to branch office network over EoIP tunnel but it doesn’t seem to work. I have a NAT on the SXT because I can’t route over APN

Simplified diagram attached.
apn.png

You need to be directly connected for the adjacency to form. Why can’t you GRE all the way to the node you want to form an adjacency with? What’s the IP of Branch Office R4.