SAPOS is a popular way of obtaining correction data for D-GNSS
(e.g. RTK). The data is distributed by SAPOS via the mobile phone network and can be received using a SIM card. This raises the question of which provider and which speed
is the right one.
The main thing to look out for here is the best possible network coverage in your area. You can find reliable information on breitbandmessung.de, where you can filter by provider, among other things.
What is the minimum speed?
As only numbers need to be transmitted, you are definitely on the safe side with a 4G-capable tariff of your choice.