@Rob-_- Sadly not, as I never had to install any 4G router, and I try as much as possible to stay away from low budget (too afraid it could fail sooner than later 😅).

Draytek is not cheap, but reliable 

  @Overtech_NC  Contacted TP-Link customer support and they really did a good job on checking the cause of the problem.

After testing some hours the conclusion was that the 4G ISP was blocking some traffic, causing this NAT forwarding problem. 

This was confirmed when I contacted my ISP and they could not do anything about it. Many 4G ISP's use Carrier-Grade NAT (CGNAT) where the ISP uses a single public IP address to serve many of their customers who all receive their own 'public' IP from the ISP. Therefore mapping will not work.

So the problem in this case is not the MR200 but the ISP.