In my country you can buy a prepaid plan without ID. Lets say I buy two 4G dongles, connect both of them to a Rasperry Pi, pipe my connection trough the first dongle to the second dongle, hide the Pi in a public building ceiling and power it from the AC-line.
Assume that nobody find the Pi, the data is encrypted, no DNS or WebRTC leakage and I'm not using any of my personal accounts (eg. bank). Is there anyway that the ISP or someone else can find out where the data originally came from (IP address etc)?