Actually the whole stuff, I believe I can somehow manage to find out a way to get a an existing Let's Encrypt certificate working in the RoS, but how to use the Ip->Cloud to get one seems to me (as much of the hotspot related stuff) largely undocumented.
The overall project (with no deadline, I have all the time in the world) is this one:
basically a "Work in progress"/"Out of order" digital sign on the LAN.
If you have some related documentation/links and/or ideas (and of course the time) please post there.