download samsung members app and submit a ticket.
once you open the app, click +, then click error report.
im sure they will be able to tell you if its the software issue, or hardware issue.
include your router information as well.
I have Google router too and I have exactly the same issue than yours!
I've try everything, global reset, etc, still disconnecting when I'm on the 2,4ghz wifi seems to not drop wifi when I'm close to the router (so when I'm connected to the 5ghz wifi)
I just purchased a S20 5G (hadthe S8 Plus with no issue) and have exact same issue on Google Wifi. Wifi connection stays up but no internet connection. Have to switch wifi on and off to get it back. No other devices have this issue. Tried all of the usual resets. Anyone found the underling cause to this issue yet, seems like it has been a reoccurring issue for some time with this phone.