To change power supply unit won't help improving power supply of the USB port. Y cable should be better if there is one. If not, I don't think it deserves to try because it doesn't solve the problem...

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