Like a popup how? What kind of dialog is it? It's more likely to be an app that's bundled by your carrier than your carrier MitM'ing ads into your stuff which is kinda what it sounded like
Caveat: if they're doing that, then they're almost certainly data mining your data streams (e.g. dns lookups etc.)
I wouldn't feel secure on such a carrier unless I also VPN'd traffic to a reputable provider (Nord, Express, or Proton) and forced DNS over TLS to known servers.
SIM cards can come with apps preloaded. There was a carrier in Mexico that would load a SIM app for Dominos Pizza and you could order a pizza from your phone if you were on that carrier. I learned this because of some carrier certification feedback I had to disposition at one job.
