Would a Pi Zero W powered by four 18650 Li-ion cells in parallel with a charging board [1], and a 4G HAT module [2], that runs a little over 24 hour continuous operation, work for your definition of "portable"? The drawback with this approach is you would own a separate SIM and plan for the 4G data. I'd also like to figure out how to cram another WiFi board on to run WiFi As WAN, but HAT stacking is hit and miss.

[1] http://www.samplerbox.org/article/howtopowerrpiwith18650

[2] https://www.pianywhere.com/product/pianywhere-4g-raspberry-p...