Problem's With Ebay Login

[#1]

Hello
We lost our broadband connection 10 days ago but the problem was resolved whatever it was around 4 days later, but ever since I've  had problem's logging into numerous websites but most notably Ebay.
Using wifi and my Tablet, with the Chrome browser. I attempt to login on Ebay, the screen goes blank and you expect to be redirected to the My Ebay page but instead the login boxes appear again, this happens time and time again without being able to login. No error message for incorrect details. 
But if use my mobile phone for tethering to provide wifi on my Tablet, again with Chrome browser I'm able to login in straightaway as normal.
Any idea's?
Thanks

[#2]

This shouldn't fix the problem, but before we go on, can you please disconnect your router and connect again. I'm wondering if a different IP will help to solve the problem.

[#3]

Have you changed router? if so you possibly have an incorrect MTU value set. I had similar problems a couple of years ago with ebay and my home banking site. Turned out to be  to high an MTU value set in the router.

[#4]

>>>  incorrect MTU value set.
That was my first thought before I even got to your post.

[#5]

Thanks for the replies.
We're still using the same router, I tried resetting unplugging the power but that didn't help.
The MTU value was set at 1500 so I reduced it to 1400 and it cured the problem,  set it back at 1500 and it's still okay. 
I did the ping test and the maximum I could set it to was 1452 without the packet needing to be fragmented, so I've added 28 for the header and set it at 1480.
I wasn't aware of what the BTU setting was for, but I do now. 
Thank You