Some notes that would have saved me some trouble.
SumUp customer service says, if you are using Loyverse on a tablet with SumUp, and you don't intend on purchasing an internet data plan (activated SIM Card), you need to use an iPad. For Android a Wi-Fi connection is not good enough. If you use an Android tablet without internet data, SumUp will inevitably stop taking payments for an indefinite period of time.
After having problems with a Galaxy tablet, I switched to an iPad and haven't