[naavis]

Would the ability to anonymously disclose vulnerabilities help? I fear that disclosures like that would just be ignored, though.

[ice3]

What one should do is to involve the www.NKSC.lt - Nation Cyber Security Center. They have a form to submit vulnerabilities

I did send a few reports from throwaway email accounts, but the issues did not get resolved.

I suspect there's another way to go about it - if you're a Telia customer - send them GDPR Article 33 request.