Nick, it is not recommended. You should read this http://forum.tp-link.com/showthread.php?78007-Declaration-of-3G-4G-compatibility-issues&region=china.

dkaswan, your modem is in compatibility list or not? http://forum.tp-link.com/showthread.php?78007-Declaration-of-3G-4G-compatibility-issues&region=china

There are two instructions which may help. http://www.tp-link.com/en/article/?faqid=636 http://www.tp-link.com/en/article/?faqid=637

You should confirm if the modem is in compatibility list of the router. Read this http://forum.tp-link.com/showthread.php?78007-Declaration-of-3G-4G-compatibility-issues&region=china.

The quality of the cable between two routers is not good... Actually, you can configure WDS on the second router, so that you can link them wirelessly. http://www.tp-link.com/en/article/?faqid=227

What's the speed if a PC is connected to one of LAN ports on the router? On the router, try to modify some settings, channel width to 40MHz and secure the network with WPA2-PSK, AES.