TV Set Top Box Ethernet Connection Issues with External Router

To ask my question I need to first describe my home configuration and the reason it is configured as it is.

About a year ago I was simply using the Comcast Gateway for both Ethernet and Wi-Fi. Everything in my house that can connect by Ethernet has a hard-wired connection, including the three TV set top boxes. I noticed Wi-Fi worked well except some single band (2.4 GHz) only Wi-Fi devices (e.g. door locks, garage door opener, thermostat, doorbell, etc.) did not remain reliably connected. Two of them would too frequently spontaneously lose their Wi-Fi link, and it required resetting them to get them to reconnect. This was not only an annoying pain, I needed to be sure I could very reliably access these devices when away from home. After a lot of research, I began to suspect the problem was related to the Comcast Gateway using the same SSID for both the 2.4 and 5 GHz Wi-Fi bands. For reasons that completely elude me, Comcast no longer allowed me to reasonably be able to change the SSIDs. So, I purchased a fast high quality/feature router, placed the Comcast Gateway in bridge mode, and established unique SSIDs for each Wi-Fi band. I do not know if the common SSID was truly the problem, or the new router just has a much better Wi-Fi implementation, but it did completely solve the problem.

Unfortunately, a new problem developed associated with the three Comcast set top boxes. They all developed connectivity problems when starting up. It turned out that this problem was associated with their Ethernet connection coming from the switch in the new router. I took the Comcast Gateway out of bridge mode but turned off all the Wi-Fi bands. This reactivated the four-port switch in the Comcast Gateway. I found if I connected the set top boxes Ethernet feeds to the Comcast Gateway switch, they played nice. If I reconnected them to the new router, they seriously misbehaved, especially when turning the TV on after it had been off for a while. So, this configuration, Comcast Gateway NOT in bridge mode but Wi-Fi turned off, is my current configuration.

I am assuming this configuration creates no security concerns but might even enhance security because now I have both the Comcast Gateway firewall and the firewall in the new router active and in series. Is this the case?

Also, any thoughts why the TV set top boxes do not play well if fed from the new router switch? Everything else fed from the new router switch works reliably and very close to the 1 GHz internet speeds I am paying for. I assume there is some obscure setting in the new router that needs to be changed but I do not have a clue what it might be.