I also have a below ground cellar. I use these below ground for my wine and above ground for my houseplants (tracking the humidity).

If your natural underground cellar isn't a perfect 58 degrees in the height of summer that is okay, as long as its dark, somewhat humid, and the temps remain stable. Remember, in real old-world cellars temps naturally rise and fall in the summer and winter. Mine gets into the mid 60s in summer and low-mid 50s in winter and I do not worry about it. Temp changes are gradual and natural.

Edit: I read too fast and saw you want something you can track on your phone. This isn't that. Sorry.

https://www.amazon.com/dp/B07QMZL448/ref=cm_sw_r_awdo_navT_a_W593GS8V9N4RSD3RZ536 this one is really good I use it and it also doubles as a thermometer