To use the TL-MR3020 as a wireless router, switch the hardware mode switch to 3G mode, then login to the router's configuration page, finish the "quick setup", choose WAN only as the internet access,...

Read this: http://www.tp-link.com/en/article/?faqid=441.

To me, I think the biggest difference between the two is the language; the function is almost the same.

I don't see Huawei E303 on the router's compatibility list www.tp-link.com/en/support/3g-comp-list/?model=TL-MR3040&location=2107. So there might be possible compatible issue.

There is no menu on the router to disable NAT. But you can connect your main router to its LAN interface and disable DHCP on the TL-MR3220, in this way the NAT is disabled on the MR3220.

There is a wifi button on the side panel of this router. I don't see a USB on/off button. Yours must be the correct device! Do you mean you want to make one USB on/off button by yourself?