@ign3u5 

Hello,

To precisely locate the current issue, please try the following suggestions: 

1. Please login to the TP-link web page as the FAQ:
Login FAQ for Archer MR6400:https://www.tp-link.com/en/support/faq/2838/

And go to Advanced-> Status --> Internet --> IP address （for MIFI: Status--> IPv4 address） to check your WAN IP address.

Please go to what'smyIP website to check the real public IP address :
https://www.whatismyip.com/

Compare the " WAN IP" and real public IP address on what'smyIP website, if they are Different, that means your ISP assigns a private IP/limited IP for 3G 4G router andISP side still have a NAT device that blocks the ports. 

Please contact your ISP and let them release a real public WAN IP for your router. 

2. If they are the same, please refer to this FAQ to check the Port forwarding settings: https://www.tp-link.com/en/support/faq/1685/

3. Please turn off the firewall on Server or host side.

If you need any further help, please help confirm the information below:

1. May I know why do you need to open ports? What is the IP and gateway of that device/ service?

2. Can you access the device/ service under the local network? How do you access it, for example by putting in the IP address in a Web page, our App? 

3. Please send us a photo of the TP-Link Status page and a photo of what'smyip. 

4. What is the Error message when you access the device/service remotely, please send us a photo of the Error. 

5. Please let us know the fimware& hardware version of the tp-link device.
https://www.tp-link.com/en/support/faq/315/

May it help and have a nice day!