Messages

I don't understand your instructions, so without detailed steps, can not follow them.

NEW INFORMATION :
I've connected the newest modem to a PC different from this - and all is well - default gateway accessible; credentials accepted.

Then I connected THIS PC to that new modem, and could not access the internet, so I reconnected THIS PC to the old modem, and am able to make this reply.

Vikram - no improvement.

Deanweb - The 'DOS' window resulting from Vikrams' suggestion states that Default Gateway is 192.168.1.1 - isn't that enough proof ?

A TP-Link modem [MODEL TD-W8901G] used to give me access to what ipconfig in CMD says is its default gateway : 192.168.1.1 After years of use it would no longer accept credentials of admin/admin. Now it will no longer give access to 192.168.1.1 - it times out - and does that in 7 different browsers.
My ISP examined the issue and recommended a new modem. That new modem (D-Link) would not accept those credentials either; and also no longer allows access to 192.168.1.1 The ISP then recommended a second new modem - same model D-Link. It would not even let me reach 192.168.1.1
My internet is quite good most of the time, so that is not an issue. There are rare times when the internet is missing - and THEN I look around at what I can tweak to get it back, but I have no access to 192.168.1.1 The problem is the same on my laptop - connected through WiFi.
My ethernet-connected desktop is an upgrade of Win7 Ultimate 32bit to Win10 v1511; my laptop is a fresh install of Win10 v1607 - also 32bit