I have done something similar a couple of years ago. I had a DSL connection that was restricted to one IP address only (the company ensured this by preventing the modem from working properly unless the network card on the PC had its MAC address match the set MAC address on the modem (yes, it used to be restrictive like this in Australia). I managed to share this internet connection by using a software which ran on the PC connected to the DSL modem, and this sofware acted as a “server” to channel internet requests made by computers on the network (my laptop, for instance) and I was accessing the internet through the main PC connected to the DSL modem.
If only I could remember the name of the software...
Maybe WinGate? I used that years ago and it worked well.