We do support using PPP over a serial UART interface. We have a user using wvdial for connectivity, it works as expected from within the container - You can checkout an example which we made for the yantrr board (Beaglebone with an onboard USB 3g modem) here, you can ignore the modem initialisation parts and reuse the wvdial components. Both USB and serial modems work with us, I do however recommend a USB hub if using a USB modem as the beaglebone sometimes has issues providing power for 3g modems.
Regarding saving data usage we have certain features that can reduce the bandwidth needed to ~2MB a month, Unfortunately we don’t have these controls exposed in the UI/API yet [We have them in the pipeline for the next month], so we will have to set them for your application from the backend. For now reducing the logs printed from your application and increasing the “Check for updates every X minutes” count to 1 day in the advanced section, while downloading an image will considerably reduce your bandwidth consumption. Do let me know if you want to try these features or have any other questions.