On what pages are you seeing this? Are you able to provide a screenshot? If your Internet drops during one of these transactions, there's a possibility the router is trying to intercept the session to present some diagnostic help in your browser (the router is manufactured by Sagemcom). If this is attempted over HTTPS, then the cert will be presented as self-signed, because it is.
Bob Pullen Plusnet Product Team If I've been helpful then please give thanks ⤵