"Site prefix" is simply the network address for your LAN – e.g. in IPv4 you'd usually configure
192.168.1.0/24 as the prefix. The main difference with IPv6, though, is that usually these prefixes are global and obtained from the ISP (often delegated via DHCPv6-PD).
You probably already know the above stuff, but it's worth repeating because it's part of why your setup doesn't work.
(As for "radvd", it's the name of popular SLAAC server software for Linux.)
I tried turning on WAN IPv6, but then it'd automatically fetch my IPv6 prefix as the address, but I think it's not supposed to do that.
No, that's exactly what it's supposed to do. The Archer expects to be talking to your ISP on the WAN side, so it configures its own IP address via SLAAC and tries to obtain a prefix for your LAN using DHCPv6-PD.
But it doesn't have the expected result because you connected the WAN port to another router – the Thomson modem isn't just a transparent modem, it's also a router and it already did the exact same thing as the Archer is trying to do.
In short, you're trying to stack two routers, each having its own LAN – Archer thinks it's talking directly to the ISP, but instead it's talking to the Thomson.
Router stacking is not a big problem in IPv6; it could work if the Thomson was able to provide a prefix to the Archer – but it looks like that doesn't happen. (Maybe the Thomson doesn't have a DHCPv6-PD server at all? Maybe it does, but the prefix it obtained from ISP is too small to sub-delegate any further?)
Switch the Thomson to pure bridge mode, so that the Archer could talk directly to the ISP. (Among other things, that means disabling Wi-Fi in Thomson.)
Do the opposite – switch the Archer to pure bridge mode, and let the Thomson serve both IPv4 DHCP and IPv6 SLAAC/DHCPv6. (The Archer can still act as an expensive Wi-Fi access point and/or Ethernet switch; it just won't perform routing.)
If you really want two separate networks, check if the ISP could provide you with a shorter prefix (e.g. a /60 would be enough for 16 subnets).
I don't want to suggest this, and I doubt the Archer is even capable of it, but… if you really want two separate networks but are short on global /64's, it's possible to use a private (ULA) prefix for the 2nd LAN, and have the 2nd router perform IPv6 NAT or NPT.