Hello
I would have checked the settings to see if 5G is switched on for the mobile function. The network coverage is also quite good in the meantime; to be on the safe side, you can check with your provider.
I have already heard that older sim cards do not work. I would therefore ask your provider for a new SIM card.
Greetings
I ordered mine on Amazon. That was shortly before Germany closed down.
But there are also shops in Switzerland that specialise in accessories for mobile phones. You might want to check there...