@bhavishsivaani
Good day.
Thank you very much for your time and patience.
May I know your model number of the USB modem?
Please also check if it is on the compatible list of the TL-MR3420: https://www.tp-link.com/en/support/3g-comp-list.html?model=TL-MR3420&hdVer=V5
Were you trying to say, the router had a unstable connection because of the USB modem?
Thank you!