https://www.tp-link.com/faq-1201.html

Port 1194 for OpenVPN

Maybe you can capture and post the sreenshot the Internet Setting page of both C60 and old D-Link router, then we can compare the setting and try to find the problem?

https://forum.tp-link.com/showthread.php?81462-How-to-recovery-the-router-when-you-bricked-it

If possible, set up the LTE Router to be bridge mode, and C5400 get the public IP runing with No-IP DDNS, problem solved :) If not, yes, you need set up port forwarding on LTE Router (better use...

http://www.tp-link.com/us/HomeCare/