Hello Experts

I have a LTE Huawei B612 Router and a Mikrotik rb951ui-2hnd. Connected Mikrotik to LTE router WAN port and notebook connected to LAN port. I only want to use the Mikrotik as a VPN PPTP server (yes I know its not secure), but the LTE doesnt allow me to setup L2TP. I can access the internet through my Mikrotik, PPTP is setup.
Test - I connected my notebook to the WAN port of Mikrotik and can connect using PPTP. Working 100%
I have setup PPTP port forwarding on my LTE router as well.

Test PPTP VPN from other internet connection(outside connection), not working. I think the LTE router is blocking me but not sure what else to do - Please help

LTE Huawei B612 must have a Public IP... must have got DMZ at MikroTik WAN port.
Maybe LTE Huawei B612 can be setup as transparent Bridge...

This looks like not related to MikroTik problem.