For me, the Zigbee ones (generic ones from AliExpress) have been great inside both fridge and freezer. Bluetooth ones occasionally dropped off (and were much worse re: battery life.)

What happen to the batteries when inside the freezer ? Sensors seems to be fine at -20°C, but is it the case for standard AAA or CRC32 batteries ?

I can't speak as to what happens with the chemistry but the Zigbee sensors do seem to be fine on standard (not rechargeable) AAA batteries at ~-18ºC.

Rechargeables were worse for me and thus I haven't used them at low temperature for a couple of years - maybe things are better now.

Yeah I thought this might be an issue but it's been running flawless for months without ever dropping out.

It does, though in my case I've purposefully put an IKEA smart plug (which acts as a Thread/Matter router) in the kitchen, too, so the signal only needs to reach 2-3 meters out.

I did the same and just added zigbee repeater outside the fridge.

Works great.

The door seal is plastic/rubber and is RF transparent.