PPTP VPN behind NAT

I have an unconventional setup where I have two Edgerouter Lite operating in my system as such.

Fiber Modem > ERL 1 > Powerbeam AC > ERL 2 > Switch.

The reason I have the first ERL is because there needs to be a guest network on that side.

I've setup a PPTP VPN on the second ERL, and would love to connect to the internal network, but even adding 1723 TCP and GRE rules on both ERL, I can't seem to connect to the VPN. Is there a way I can solve this?

Basically I am trying to open up a VPN tunnel from the outside world to my second ERL.

Thanks in advance