Toast POS Modem

We have purchased a food truck that has a Toast POS system that came with it. We contacted Toast and they said they will send us a new router but they said we will need a 4G/5G modem that has an ethernet port connected to it.

We got in contact with AT&T and told them that we need a modem but it seems like they didn't really understand. They gave us a SIM card that I believe is a new phone line that we can use for the modem. We went to Best Buy and Walmart to look for a modem but they seemed to not know what we were talking about.

So we need to look for a modem that has a place we can put in a SIM card to travel, I guess. We are very new at this and don't quite understand anything about mobile wifi.