Plusnet wifi router 2704 and incompatibility with older Apple Mac OS

[#1]

Anyone out there still using Apple Mac OIS 10.6.8 or older and having problems with interference from Plusnet's latest wifi router for fibre? I work on an Apple iMac which I have split the drive and can revert to this older OS in order to work in some older softwares, in particular Macromedia Freehand. I have encountered major problems - in fact it more or less totally incapacitates working on most softwares and more often than not just locks the whole computer up. Took me a while to realise it was the router that was the problem, having taken my Apple MacBook, which I also have on OS 10.6, in to a local computer shop to see if the problem could be fixed - in their premises of course it worked perfectly. Back home I tried using the laptop in a room most remote from the router, which helped, but after switching the router off completely, magic, everything worked perfectly on desktop and laptop.
Suggest to anyone out there thinking of going to super fast fibre/getting a new router - if of course there is anyone still using such old OS! - to think twice first and check stuff out.
I guess I need to raise this issue with Plusnet to see if they have another router that won't interfere!

[#2]

Saw this reported not that long ago elsewhere and there was a simple fix for it but unfortunately can't remember what it was, fairly sure it was a general problem rather than anything to do with Plusnet or their equipment. Fairly sure Google will be your friend.

[#3]

Try disabling IPv6 on the iMac

[#4]

I've been stumped by this problem for a couple of weeks so thank you pwatson for solving it - disabling IPv6 definitely works and I'm now a happy bunny. Here's how to do it:
https://www.youtube.com/watch?v=_iXoMNj0fAE

[#5]

This YouTube clip seems to have worked for me.

[#6]

So pleased to have found this query and solution.  Just moved over to Plusnet after years of dwindling service from BT and have spent two weeks trying to persuade my beloved 10 year old MacBookPro to accept the change.  iPad switched without me even realising but MBP stopped dead at home but was happy at work and elsewhere with wifi - even in the Apple Store when I really thought I was going to have to trade up to a new model.
Thanks again for the solution - MBP lives to see another day!