Xfinity: modem and cable vox from same cable port?

[Bigbacon]

Is this possible? Where my parents modem is, the wifi single sucks unless you are less than like 10 feet away. Dont want to run coax some where else in the house. Only other jack is in the basement.

 

[Krispy Kritter]

Depends on the signal level. A splitter will reduce the signal level, so if it's marginal or low already, you will create issues.

Also, I'm not familiar with Xfinity, but Charter for instance, used to run a dedicated line to the modem and a second line for TV/phone service. It's not standard practice any longer, but again, it all boils down to the current signal levels.