Bottom line, if you have a router and there is a vacant LAN port on his existing router you can plug cable into it and the other end of that cable into the WAN port on your router and you are good to go. If all his LAN ports are filled you can unplug a cable from one port and plug it into a LAN port on your router, then use that now vacant port to connect your router.
Not so bottom line, there are two different functions, a modem, that makes the subscriber connection to the provider. A modem will have two types of connections, typically a coax connection (or phone jack) and one computer jack.
A Router is a separate function, and will have multiple computer jacks that all look similar, with one marked WAN and several marked LAN.
If the modem has one cable input and multiple computer LAN outputs it is a combination modem and router.
If you have a modem/router you would need a separate subscriber connection and subcription. It would be rare and complicated to get two separate modems connecting to the provider in a single house, but if you pay for it the provider will probably do it.