Those mobile/wifi hotspots have their own internal batteries (at least any of the ones I’ve had – due to the Isle of Man not being in the EU, we have to pay eye watering roaming charges for data in Europe (including the UK) and my solution for years was to carry a mobile hotspot with a local EU sim in it. These days I just use the Android phone I use for Skydemon, with a UK vodafone SIM in it. With Brexit, I’ll probably have to go back to carrying multiple SIMs or a hotspot, grr)
If you just need to signal an emergency, then a PLB would be the best solution I reckon.
If you just want to be able to talk in a non-emergency, and have a drone already (say, for photography), then lofting a hotspot up a couple of hundred m is probably not such an awful idea.
I had an E587 for years
Eventually it didn’t get used because the roaming contracts made shopping for local SIMs (always the high point of any trip to Croatia or Greece 
) irrelevant.
Unfortunately these need a SIM card which is yet another thing to keep going. One way or another (PAYG or contract) this is a big hassle. I am paying £9/month for a Voda contract SIM in my tablet, which also acts as a backup for my phone (£15/month). PAYG expires after 90 days (on most UK cellulars, with a few exceptions*) and has become a total hassle.
* Deals like the old Virgin Mobile PAYG SIM which topped off automatically via a direct debit, and de facto never expired, still work, but Virgin shafted their users by blocking data on them, so they work only for voice and sms.
