It sounds too good to be true... A sim that can be purchased anonymously and used anywhere... What am I missing here? Cellular service is inherently tied to physical infrastructure - I'd expect governments to make use of that fact. It's much easier to enforce than KYC.
So, honeypot was my first thought. On the other hand, if you're only using data and not voice/sms, and everything is end-to-end encrypted, what can they really find out about you, other than your physical location?
I live in South-East Asia and I connect through my glinet router to other networks:
https://www.gl-inet.com/products/gl-axt1800/
It runs with openwrt and a customized UI available. Depending on the country though, it can be hard to connect. Typically in Japan ISPs use a specific way (called DS-Lite) to connect from ipv4 to ipv6 networks and the implementation in openwrt has almost never been tested.
In other countries it works well, I use it from a LAN port or as relay.
In retrospective I should have spent more money on a router to get better coverage through walls.
mudi gl 750 is a vpn dongle that you can combine with blue merle to switch the imei every time you put in a new (local) data sim. It comes with an RJ45, and can also relay from local networks.
never put a sim card, or e-sim in your phone - else you are easily trackable and geolocated.
Local sim card with internet plan works great. You can usually get them right there at the airport.
If you connect to a shared WiFi remember to use a VPN to reduce the snooping from others into your device. It's a good idea to have a VPN anyway.
I've been having a look at a more worldwide solution, like international sim cards, or even satellite based internet, but the last time I checked, grabbing a local sim card was still the best in terms of convenience and price. Things might have changed recently though, so happy to hear comments about a global solution.