PPPoE server, dual stack IPv4+IPv6, buggy SOHO routers that refuse IPv6

I’ve been running a small local ISP, running VyOS on 4 boxes (2 as BGP routers, 2 as PPPoE servers, OSPF between them). IPv4 setup has been running for a long time, mostly without issues. Now, time to break something and implement IPv6…
It works when set up properly with a MikroTik as PPPoE client, but I have trouble with some cheap routers (mainly Phicomm KE2M - manufacturer no longer exists, too cheap to be upgradable to OpenWRT). IPv4-only setup works with “set service pppoe-server ppp-options ipv6 deny”, but when I change it to “allow”, these broken routers keep disconnecting a few seconds after successful authentication, and RADIUS logs shows these disconnects as User-Request.
Because of this, I may need to run separate PPPoE servers on separate VLANs for broken and non-broken clients. Is there a better way? Since the disconnect happens after authentication, perhaps it is possible to aply “ppp-options ipv6 deny/allow” selectively depending on the user (so it can only be enabled for those with known good routers)?

Hello @marekm72
Try using delegation-prefix and ipv6 address (for example):

set service pppoe-server client-ipv6-pool delegate '2001:db8:8003::/48' delegation-prefix '56'   (ipv6 prefix)
set service pppoe-server client-ipv6-pool prefix '2001:db8:8002::/48' mask '64'                  (ipv6 address)

Documentation:
https://docs.vyos.io/en/equuleus/configuration/service/pppoe-server.html#dual-stack-ipv4-ipv6-provisioning-with-prefix-delegation

Actually I use RADIUS attributes for that (Framed- and Delegated-IPv6-Prefix) and with a properly set up good client router it works (with MikroTik I was able to get 10/10 on test-ipv6.com). The problem is, the customers have their own routers not under my control, a mix of different routers with different bugs. All they have in common is that they support PPPoE for IPv4, as that is the service I currently provide (each customer gets a single static IPv4 address, assigned by RADIUS based on PPPoE username). Now, I want to add IPv6 (static framed /64 and delegated /56 prefixes for each customer) for those with routers that support it. But, there are some problematic ones that don’t really support IPv6, but break when ppp-options “ipv6 deny” is changed to “ipv6 allow”. I will try to capture some traffic between such a problematic router and the PPPoE server, perhaps it will help to understand better the issue.