Hey there katrian,
The SOS indicates that your device is not connected to a network, but that it can make emergency calls if needed. It sounds like this has occurred on two different devices. Because of this, it is potentially an issue with your carrier, as they are the ones who provide the access to their mobile network.
That said, we absolutely want to do our due diligence and make sure there is nothing wrong with the iPhone itself. To do that, we can follow the troubleshooting guidance from the Apple Support article If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support. It addresses these exact issues, and we would recommend following all of the steps outlined in it.
Should the issue continue after that, we can recommend getting in touch with Apple Support directly. They will have access to the tools and resources necessary to further investigate what may be happening. You can reach them here: Get Support.
Have a lovely weekend.