I am not understanding how do you want to configure this TL-WR702N, in access point mode? repeater or client mode?

What's the model number of your USB dongle? Is it on the router's compatibility list www.tp-link.com/en/support/3g-comp-list/?model=TL-MR3420&location=1151? Refer to the troubleshooting guide:...

It seems you failed to access the router's configuration page. First, pick up the default gateway address by ipconfig; then open IE and try to access the router with the default gateway address.

I found the router's compatibility list says only ZTE AC2787 V1 is supported by this router: www.tp-link.com/en/support/3g-comp-list/?model=TL-MR3420&location=1151. Is your AC2787 modem V1 or what...

Yes it says ZTE AC2737 is compatible with the TL-MR3040 on the router's compatibility list. But..... You'd better send an email to report this problem to support@tp-link.com Maybe, TPLINK support...

Access the router's configuration page, and check if you get the WAN IP address and other details on the Status screen. If not, the router is not configured properly. If yes, go to system tools->...