Report the problem in details here and TP-LINK people will help you to do troubleshooting. http://ticket.tp-link.com/index.php?/Tickets/Submit

Did you Enable "Allow Guests to access my Local Network"? As W9980 works in AP mode, if it is not enabled, W9980 won't be able to communicate with the main router....

Network Type supported by M7350 is listed as below. Not sure if any ISP in Croatia would work. You'd better confirm with local ISPs in Croatia. FDD-LTE Cat4 (800/900/1800/2100/2600MHz)...

Someone said he was provided with a specific firmware by Techsupport and after that applying the credentials manually, it worked....

I didn't see a way to restart the router without power-off. Try to upgrade the latest firmware and see if the connection will be more stable.

As I know, the bug has been fixed with firmware version 140912. Can you try that firmware to see what happens? TL-MR3420_V2_140912 5.Fix the bug that the backup link won't revert to the main link...