Okay, it looks like I have some research to do, see if I can get this figured out, or at the very least start to understand what's going on.
I very much appreciate your help, though! Much obliged!
FYI, although this is old, most likely this is the problem:
"A quick update to this thread if anybody is interested. It would appear that TP-Link devices do not support /56 Prefix delegation which BT use. I believe they are working on a fix for later models but not sure at this time if the 9980 will be included."
That has been my experience with most home routers. Unfortunately this is the case of the router manufacturer supporting only a delegated prefix size of /64 which actually is against all recommendations - every ISP is supposed to provide their customers with a /56 or a /48 according to the standards, and almost all do, but almost every router vendor only supports prefix delegation with /64 which is wrong and a very bad implementation. It is a case of the router vendor not doing their homework and therefore only supporting IPv6 in a way contrary to all recommendations, and not testing it properly, because they just don't care.
See if you can upgrade the firmware to something that supports prefix delegation for /56 or /48 instead of just /64. What should happen in a properly functioning router that actually implements the spec correctly is the router should not freak out if given a /56 or /48, instead it should simply accept it and take a /64 out of that and assign it to the LAN port, maybe a second /64 for guest network if you have that, and null route the rest.
Because the home router support is spotty at best right now, it is safest to go to a semi-pro router like MikroTik (like I use) or Ubiquiti, where you are guaranteed the IPv6 prefix delegation is going to work because they actually know how IPv6 is supposed to work. The only downside is there are a lot more configuration options, so it is easier to get lost in the interface.
First however see if you can contact TP-Link and get a firmware that works with the /56 prefix delegation, even if it is a beta version, to avoid you having to purchase something new. Unfortunately their support chat is probably not going to be super helpful because most of the front-line support people have a very poor understanding of IPv6 and are incapable of helping due to that.
Yes you can make it work in bridge mode, I've been doing it for over a year with an Ubiquiti EdgeRouter (3 then 6P).
You need to have your router use a DHCPv6 client, and can request a /64 or /56 (IIRC?). For my setup, I then hand out IPv6 addresses to client machines using SLAAC from my EdgeRouter.
If anyone has a Ubnt, I can help them out.
Shaw actually supports handing out a /64, and then the TP-Link should hand out /128 to client devices over SLAAC, if their device works correctly.
As for prefix delegation? Hmmm... not so sure. I don't do it on my device at home.
You surely must be using DHCPv6 prefix delegation at home. Without it, your router can only get an IP on the WAN port. It wouldn't make any sense to get IPv6 for your router only and not for your internal systems.