What I do not understand is what exaclty this chap is trying to tell me - change my router password from the default of "admin"?
That's why I had advised you (in the Topic where you originally asked) to lookup the make & model of your router & post back. If you have that, a quick Google search will provide the manual that shows these settings. We can also assist if we know not only the make, also the model of your router.
Scattering the same question across multiple Topics only creates confusion for those whom assists you, and you also get an answer quicker by keeping all together in one Topic.
There's at least three Topics where you've sought assistance for the same issue.
This is a network issue, not a OS one, as it apples to all of the computers on your network, be it Linux, Windows, Mac or Android, not a Linux Mint one. My example of the Linksys EA4500 was such because it's the one that I have. Yet it was only that, an example.
Note also that if the router is supplied by your ISP, you'll have to get permission from them to make changes. For example, they'll want to keep Remote Administration enabled for their purposes.
I provided a very detailed list of changes here to be performed in Post #25 here. Please keep in mind that the scattering of these questions makes it harder to find the prior answers given. Keeping this in one place makes things so much easier & allows more time for us to assist others in need, I spent at least 10 minutes to find the content below.