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Does this make my router/laptop vulnerable to attack from a hacker who knows my default login details?
There was an option within settings on the previous version of this router to renew the security certificate I think, which I stupidly pressed one day and the connection has been insecure ever since. Some strange things have been happening to my laptop while I'm using it, like pages I'm reading on the internet being replaced by huge image files and the network connectivity always seems to be running high in peaks even when I have no applications open. I feel as if someone has hacked my PC and wondering if this insecure connection could be part of the issue?
The setting in the router that you are describing is often there to decide how you view the router setup pages from your local connection that is all. And it's often problematic because if the certificate is out of date you get a (bypassable) error page even when viewing the setup pages.
Those setup pages are not viewable from outside of your local network unless you have a compromised machine within your network!
The problem is, that it sounds as though you may have a compromised browser or even machine. Your first step should be to remove any random browser extensions you've installed and run an updated Anti-Virus.
Try that for now, and let us know how it goes!
Personally I use the Brave Browser for most of my Cerfing.
I have not seen the option to use http rather than https, but if it was the router I would do a pinhole reset and start again.
Run Malwarebytes or similar.
Your picture is awaiting moderation, so I am just guessing.
It's down to the server as to whether it uses HTTPS or HTTP, but a user can set their browser settings (it may no longer be possible to turn off the warning on some browsers) such that they will be warned if a HTTP page is served.