My guess is that’s its your router. I play on WiFi and it’s generally great. Last week I changes my WAP in one side of the house and now I occasionally get a similar message - never gets as far as dropping off but says it’s unstable and will disconnect if the problem persists....so for me, it was an issue with the router.
I’d say it’s wifi too and tbh google should warn people just how crappy the 5ghz band is, thought I knew my stuff about internet but got no real knowledge about advancements the last 5 year or so and pretty much assumed the higher ghz band was better than the lower in every way till it can barely reach my Chromecast about 3 metres away from router but with 2 doors in the way.
I have the same issues; playing stadia on my S10+ gives me connection errors all the time (does not matter how far from wifi) while playing on my Chromebook (ASUS) w/o problems; both connected to the same wifi. My Wifi is an OnHub-GoogleWifi mesh network, with playing priority in wifi app turned on.
Shame on Google, if their OWN wifi devices give wifi network problems, when using stadia. BTW: connection test of Stadia gives 'excellent' connection.
Generally the 5Ghz band will be a faster connection but it has less range than 2.4Ghz. They're not really so much "one is crappier than the other" but both have pros and cons. Not all devices will work on 5Ghz but if they can and are in the same room (for example) you would get a faster overall connection.
OnHug-GoogleWifi mesh does not allow to change 2.4 and 6 Ghz, but according to the google wifi app is my phone connected on the GHz channel -and giving unstable connection errors.
EDIT: I was really desperate about this lag issue, so did a factory reset of my phone; with all apps re-installing (quite a job!) -and it has resolved my issue! Obviously NOT a stadia/ wifi problem, but some unwanted settings on my phone. Everybody with similar issues; my advice is to do a factory reset (baah..).
If you are using wifi try to plug in Ethernet cable.
If you are using Ethernet, try to disable all extensions in chrome (such as adblocker etc.) and use only the stadia+ extension for stadia. Alternatively you can use the canary browser which disables all extensions right away.