No, the coax network is an RF signal, which "just happens to be" in the same band as the cable modem internet, which mean one or the other.
I can appreciate your frustration, but may I suggest just one more try. I have a total in home system versus an apartment setup, but I had a similar problem when I disconnected and reconnected our bedroom DVR. (Actually the original installer had the same problem and this is where I got the correction method -- straight from DirecTV installer technical support.)
First, I would remove the filter that the installer inserted. You can always reinsert it if I am wrong, but I think that is killing your internet. Now, the fix.
Unplug both your DVR and your receiver. Then plug in your DVR and let it go through its setup completely (about 3 to 4 minutes). When that process is complete and you have a picture, plug in your receiver and let it go through its setup. Once this is complete everything should be reset and work OK. This seems to be the only complete way to reset the IP addressing on the Whole Home DVR process. It has worked twice for me.
The bandstop filter in this case is needed, otherwise other apartments may be part of the coax network. This is a BIG DEAL with MDU systems.
"Also" the splitter he was using [and why it had to be replaced] had way too much loss for the receivers to be able to communicate.
His problem wasn't anything to do with IP addresses, but "merely" the DECA signal had no path between the receivers.