If i use the V1 english firmware version on my V2 Archer C7 wifi routere, will it work or not? Of course not because v1 or v2 it is hardware version of that router and you can't exchange firmware

Update to - ftp://tplink-forum.pl/orgin_bez_boot/mr3420v2_en_3_13_27_up(121011).bin Firmware without bootloader for your hardware version

Most of AC750 routers with USB doesn't support 3G/4G modems. You can see it in technical specification.

Can you check "Enable AP Isolation" function?

Please speak English here.

You can try with english firmware but you must regard for version of this firmware v1 or v2 or v3