@KASK999 

Hi, May I know the error message when you connect your modem to WAN port of the MR3620?
And how about the Internet LED status on MR3620? the MR3620 is on wireless router mode, right? please let me know the troubleshooting you've done.

In addition, here are answers to your questions: 
1. Please check if internet LED is light up when you connect your modem to WAN port of the MR3620, make sure the Ethernet cable and your modem are both working fine.
You could also connect WAN port and LAN port of the router itself to see if internet LED will be orange on, if so, the ports should be working. However, if Internet LED and LAN LED are both off, I am afraid the WAN port should be defective.

2. Please contact the local technical support team to confirm it:
https://www.tp-link.com/in/support/contact-technical-support/#Hotline-Support

3. No, there is only one WAN port on MR3620, other LAN ports cannot be used as WAN port.
If you have another router in the front, you could consider to use the MR3620 as an access point by connecting LAN port only(change LAN IP address manually and also disable DHCP server on MR3620.)