Found that on a web page of my provider (MMS & surfinställningar 📡💻 | Hallons forum). So under Settings → System → Mobile network SIM1 → MMS access point → Proxy address, I changed from 172.16.53.11 to mmsproxy.tre.se. However, the web page also lists MCC, MNC and APN Type, which I don’t know how to check/set in Sailfish.