Hindsight is 20/20 but I correctly guessed the ending as soon as that information was added.
After a couple decades of debugging various internet issues the first thing I now do is check the 'source' works (i.e. plug a laptop into the modem directly, but with a different cable). If that works I go down 'the line' until something does not work. That usually finds the culprit quite quickly (and also stops me from messing with my router config when it's an ISP issue).
In OPs scenario, the moment you realise that the office internet is working fine and it's only the home internet that is having issues, the connection between the two would have been the obvious place to look next.
That being said, it's still a fun story, and still quite 'unexpected' that rain could be the determinating factor on whether you'll have working internet or not.
At first (without the full context) I thought it was because rain blocked the external signal interference that was making the AP channel look busy.