Never use someone else's wallet to hold your money.
Keep the seed phrase safe. Use actual dice to generate it. Put a chunk of it into your wallet, a chunk into your password manager, a chunk in your attic or in a book, etc. Never type it into an internet connected computer.
Oh and, never tell anyone about how much crypto you have.
This is reasonably safe, and reasonably within the capabilities of a non cryptography nerd.
Maybe the sim card is on a timer to broadcast your keys after a year, so you just can't detect cellular signal usage now.
To securely own bitcoin you have to first create a new chip fabrication plant to then make your own hardware wallet.
The idea that someone would have to buy a hardware wallet to be safe, but first audit the hardware and software is both a but ridiculous and infeasible. Unless one can actually inspect the code running on the device, auditing am open source library online doesn't tell you what is running on your device.