Using AppleTV with 2 home networks?

The primary problem I’m having is that my devices are self-assigning IP addresses.

Here’s my set up:

I essentially have 2 networks in my house.

Network 1: In the basement I have a Comcast cable modem. This feeds into a wireless router. I’ve had used this set up for several months now and haven’t had any major issues. The router is connected to a switch that feeds our smart TV (Visio) and our AppleTV3

Network 2: I have another cable modem (one I own, a Motorola SB6121) on the ground floor of my house. This feeds into an internet switch (and granted, it’s an older switch a D-Link DSS-5+. I have another router that I tried as well here but the issue persisted. It’s of a similar vintage as the switch, however). This switch is connected to our smartTV (Visio) and Apple TV3 on the ground level.

The devices in the basement all work fine. When devices are connected to the wifi they (usually) work. I set up the second network on the ground floor because the home sharing for the AppleTV cuts out all the time on WiFi, but the hardwired device in the basement has no issues. I couldn’t easily run a CAT5 cable to this section of the house, but there was a coaxial outlet (the cable box is here) so I just split the signal and hooked up a second cable modem.

Now, the problem is that Network 2 devices are self assigning IP addresses. I made sure the cable modem for Network 2 was set up with my ISP and when I plug my laptop directly into the cable modem it works fine. However, when I connect the switch my ethernet connection on my laptop goes yellow for self-assigned IP and neither the smartTV nor the AppleTV work. I’ve read elsewhere that it may be an IPv6 issue, and I tried turning that off on my laptop (running wifi on network 1), but it didn’t help anything on network 2. I thought the issue may be the switch but I’ve tried plugging the smartTV and the Apple TV directly into the cable modem (SB6121) too. Doing so does not help either and they either do not connect at all or self-assign and are virtually useless.

I do not want to have to rely on WiFi because it’s already shown to be spotty for the connected devices on the ground floor, hence the construction of network 2. I don’t mind sinking a few bucks into a new switch but don’t know if that’s the right course of action because the issue persists when some devices are plugged directly into the modem. Big a gigabit switch that supports IPv6 might also be the panacea for this issue. I appreciate any helpful suggestions.

Now that I think about it two devices need to be on the same network for home sharing to work, right? Is that the issue? If so, is there a way to overcome it?

It still doesn’t explain why the devices don’t work when plugged directly into the cable modem.