Sounds like your carrier is abusing STK to display ads.

See https://www.browserstack.com/guide/stop-popup-messages-in-an...

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.