there is really no open "good" supl server and that is the reason we do not set it by default and it is left to the user to enable. some carriers have some but they are mostly just a frontend for supl.google.com. here is a list of supl servers i know of but keep in mind, there is no open source server out there. these are all run by big tech companies

supl.attmex.mx:7275

location2.kddi.ne.jp:7275

wpde.nate.com:7275

supl2019.lbs.sprint.com:7275

supl.geo.t-mobile.com:7275

and last but not least the most functional server owned by google

supl.google.com:7275

i do not know if anything other than the google one is functional so you may need to do some research there.