@MarkyR74 I should add the topology, very simple, ONT to TP Link Deco which is then using Ethernet to connect to my second Deco. I shall be receiving my ISP's router this week so will set it up and check its IPV6 settings and if it's working. 
 

I have tested with my iPhones and iPads, Steam Deck and my works Windows Laptop, the laptop gets 1 Link-Local IPv6 address and 2 IPv6 DNS server addresses and 2 IPv6 addresses. But I don't think I can test for IPv6 as it's in a VPN.

  @David-TP Hi, thanks I'll look at that link. I just have the ONT and then an Ethernet cable from that directly to the Deco. Nothing else is connected in between and the fiber line goes directly into the ONT from outside the house. It just seems odd. I have e the ISP router coming to me so will test with that once it arrives. They support IPV6.

  @David-TP Fixed it! Very odd, I set the Assigned Type to SLACC+Stateless and the 'Get IPv6 Address' to Stateful and it worked, but I know the SLACC is wrong so I changed the Assigned Type back to DHCPv6 and left the other setting on Stateful and it still worked, I rebooted the router after each change, it is strange as I'm sure I tried the Stateful setting yesterday? But all the IPv6 tests are passing now so happy days.

Take it back, it's now not working again? No reboot or anything? What is going on? Is the router or my ISP refreshing the IPV6 address and causing It to stop working?

  @MarkyR74 From what I can surmise, I've tried literally every IPV6 configuration in the TP Link app, I have had to reboot the Deco's every time I make a change, after which I get IPV6 fine for about 40 minutes ish, and with no warning or noticeable changes IPV6 just stops working? I know this by running those online IPV6 tests. 
 

I had these Decis with my old VDSL connection and IPV6 worked perfectly fine on a PPPOE connection, only thing I havn't done is do a factory reset and set them up from scratch, is it worth doing that?

  @David-TP I haven't tried gong direct to the ONT but can try to, but my providers own router will arrive today so I'll set it up later and test with that as if it still does it I know it'll be outside my equipment. My provider is someone called Squirrel Internet, I'm in the UK.

  @David-TP Well I plugged my providers router in, a Zyxel model, it connected up and I have had all IPV6 tests passing perfectly fine for nearly an hour now, no issues. Also it feels like it's running faster too, and it isn't blocking ICMP unlike the TP Link which does. So it looks like the TP Link is doing something to drop IPV6 on my network after 30 minutes if I get it working. 
Can you explain why and how to fix it? Or will a firmware update be needed?

  @David-TP Well I factory reset the Deco and it was exactly the same, it worked fine on IPV6 for 30 minutes then stopped. Exactly the same as before, do you have any suggestions of what to try? It is setup as per my providers instructions. Otherwise I'll have to send them back for a refund and buy something else.