Connect USB to VPN

I have a VPN service on Fiber Optics my a major carrier. I also have contracts with clients and have provided them 4G USBs from the same carrier. However, my clients want unlimited data service on their USB but almost all major carriers in US have stopped unlimited service. I will prefer to point their USBs to my VPN service after they exhaust their 4G usage. Can I do that? If yes, please advise how. You may send your answer to my email at zakpesh@yahoo.com.

Thank you.