Double router LAN-WAN causes connection/ buffering issues

I have been running a LAN-WAN setup for years without any major issues, but recently changed the second router, and now I'm getting some connection issues. Why I want LAN-WAN: I like having my local network separate as it helps keep devices like chromecasts separate from my housemates. The problem: The ethernet connection to my desktop works without any hiccups. However, the wifi connection to my phone and chromecast cause severe loading issues in some apps. Youtube revanced does not load, or takes multiple minutes to load a video, for instance. Same with Smart tube on my chromecast. Regular y

Question by K65 on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.