FWIW, Amazon deprecated mobi files recently and epub is the new “sideload” standard. You still have to email the file to the kindle address to be able to read them, or convert to azw3.
If you’re already using Calibre, check out Calibre-Web, which essentially uses a Calibre database as the back end. The interface is so much nicer than Calibre.
My Gen 2 only had whispernet, which relied on the Sprint EVDO network, both of which no longer exist (the company and the network type).