Solar Panel for Sigfox

Hi There.
A client needs to deploy several sigfox devices along a water pipe to monitor it, and as they would be in the open field I eas considering small solar panels to keep the batteries up and avoid replacing dry batteries

My concern is about battery type, as from what I read you can't use a LiPo (3.7->4.2v) battery cell, how would you deal with it? Vin input needs 5-6V according to the official documentation.

Has anyone here tried a SigFox with LiPO battery and a solar panel?

Maybe a DC-DC converter would help but I was trying to get my set up as minimal as possible as I would have to replicate it several dozen times...

Thanks

What about using voltage regulator and feeding through green screw (3.3V input)?

Yes but voltage regulators are usualy a good waste of power. In such a case I would prefer a DC-DC converter which are closer to 90-95% performance. Anyway I was looking for something more compact if possible to save boards and wiring. I've seen forums stating that the +3V green plug works well with >3v but I don't really know if that's safe and how much more than 3v I can try