Probably way off target but there isn't a lamp post mounted cctv camera
in the road outside is there? A lot of these use 2.4Ghz to transmit
their pictures back and could cause blocking of your system, they run a
lot more power than you. Along similar lines there are plenty of 2.4Ghz
cameras around that people put in odd places for cctv use. I know a bit
about cctv but not wifi so this is only speculation.
When you have tried everything how about moving the kit to another
location all together, work maybe, and seeing how it behaves there?


--
Bill