You can use an sd card like this post: Flashing an Electron or Photon from an SD card. Simply use the softAP protocol to transfer the new firmware one piece at a time onto the SD card, and then use that library to perform the update.
But also, have you considered an Electron? cell connection could be only used for OTA. Not sure what exactly your product is so I can’t comment on what “hijacking” means for you but there may be out of the box solutions for your greater problems here.