One of those outputs will be the one that goes to the cable modem. Since you identified that the distribution amp is powered from the same physical location as the cable modem, it shouldn't be to hard to identify the coax at the amp that is being used for the internet service to the modem. That's going to be either the VOIP/MODEM (most likely) or the #1 port.
1) I don't have the installation manual for your amp, so I'm extrapolating some.
2) If the power supply is on the VOIP/DATA port, it will need to be moved to the circuit that connects to port #1. You will have to find it.
3) Disconnect the incoming cableco line from the amp's input and, using an F81 coupler, connect it to the line removed from the VOIP/DATA port. Make certain #2 is done first.
4) The antenna signal may then be fed into the input port of the amp and you'll be right in the ballpark.
Can you get the exact model number of the amp? I can't make it out in your first photo.