Here's what you need to do:
Step 1: buy an old Linux Thinkpad, libre it w/ Coreboot, then install, say, Gentoo GNU/Linux w/ openrc daemon mode and Sway. Then turn it into a makeshift Linux server by taking 5 raspberry pi 5's, then plug them into a TP-Link JetStream T1500G-10TT via ethernet, and plug your laptop into it also. You could simply buy a Xeon server tower PC running Linux instead, but that tends to be on the expensive end. Use the ThinkPad as the control plane (running Osmo components) and use the Pis as data planes.
Step 2: buy an Ovrdrive USB flash drive and flash the LUKS2 decryption key to unlock the desktop, after using LUKS2 for FDE (Full Disk Encryption).
Step 3: take two LimeSDR minis w/ LMS7002M SDR transceiver and antennas for RF frontend - one for RX, one for TX. Plug it into your laptop. This'll connect your laptop to the cellular network.
Step 4: buy a sysmousim developer SIM card, pop it into a USB SIM reader, and plug it into your laptop to program it for your custom network. Sysmocom provides excellent open-source tooling (sysmodown, osmo-sim-tool) for programming SIMs. Ensure you get the right SIM toolkit (STK) profile for your network.
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