I was advised to ask this here (also asked on dslreports). I see my issue is not exclusive. I have had a G1100 router and wcb6200q wi-fi extender for awhile now. Not really many issues, although I've found that streaming through the extender is sometimes hit or miss. Anyway, all of a sudden over the last few days, I have all kinds of issues with 5Ghz via the extender. Devices are connected, but no internet traffic, can't even ping the router! 2.4Ghz is working fine.
User on DSLR suggested I turn SON on (had been off). Another suggested I "disable DFS on the 5g radio."