Andy Burns wrote
Rod Speed wrote

What's available if you want full wifi access at a low cost for permanent
temperature sensors for stuff like fridges and freezers ?

ESP826

Yeah, forgot about those, thanks for that.

the letterbox is all metal so the wifi can't easily get out.

If there's a slot for letters to get in,

There isnt. Its a big metal box, big enough to
take a boxed set of LP records and quite a bit
thicker boxes, with a hinged lid on top, no slot.
Stands vertically with boxed LP records that it
never gets anymore.

And our letterboxes are outside, not in the front door.

quite likely wifi will get out

Likely better to have it under the box with a plastic
cover, plenty of solid plastic boxes like that. One the
same size as the bottom of the box would work well.

With the back gate a Hue motion sensor under
there should work fine and would allow it to
be moved around to get the best location.

Cant do that with the letterbox, its on the wing wall
of the house 10' away from the eaves. Tho it might
be worth trying one on the patio door there, they
do work fine that far away inside the house and
would be less vulnerable to deliberate damage.