@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...

@Overtech_NC Thanks for your reply. Any suggestions for a low budget 4G router that works properly when it comes to NAT?

@Overtech_NC Same problem here, using an Archer MR200 4G router. Turned off all under Security. Turned off all under ALG. Turned off uPnP. Under NAT, Virtual servers, routed all necessary ports...