If Asus agree that the device is faulty then discussion here about the
nature of the fault is irrelevant.
It might have helped to have had a document from Asus confirming that
the device is faulty and instructing the seller to replace it.
But "a new router is in stock at my local store" and "ordered a new one"
suggests that PC World have a useless stock control system.
Given that you allowed them to order a replacement and gave them the
faulty unit you're in no position to negotiate - unless and until they
fail to supply you with the replacement. Also ...
<
https://www.which.co.uk/consumer-rights/regulation/consumer-rights-act>
... explains the law. The retailer's warranty is only worth pursuing if
it gives you better terms than the Consumer Rights Act. Assuming you
have had the device for more than 30 days and you can show that it is
"of unsatisfactory quality" - the document from Asus confirming the
fault should help with this - then the retailer's obligations are
"repair or replace" with the exceptions shown in the link.
I doubt that you would get any better service from Amazon. You
certainly could not talk to anybody about it!
Probably you should choose a reputable supplier in the future.
Some years ago when I ran a small business I bought several PCs
manufactured by Asus, for resale to customers. The all failed within
about 18 months with slightly different symptoms. I suspected failing
motherboard components. So I learned to stay well clear of anything
made by Asus.
--
Graham J