One of these?
These all shipped with ethernet, but the modem was optional. If it is one of these, and only has one modem-like port, then it's most likely actually an ethernet port. It's an easy enough mistake to make when you don't have both there to compare sizes. The install tech could have made that error. But the definitive way to check is to see what you can plug in to it.
Any Mac with ethernet should wire to any modern router without any problems so far as I'm aware.