it seems that C9 doesn't support dfs so band 2 or 3 are not open. But usually most devices support band 1 so it is enough for daily use.

do you bridge two extender together? it is not suggested to do that. But range extenders can connect to main router directly. like this: main router (wifi) extender 1 (wifi) extender 2 bad extender 1...

no heard of that, but you can disconnect other clients and keep just one then try again.

i think Build 20161025 is newer then the one 161018, so we can't upgrade it.

what is the current firmware version now? can you access https site with direct connection to the modem? make sure this problem is not caused by modem or isp side first.

For V1, there is only one firmware available.