L2TP ppp-options ipv6 does not work without ipv6 pool, but it should as we can get such addresses via RADIUS attributes:
`Framed-IPv6-Prefix` and `Delegated-IPv6-Prefix`
Example of config
```
set vpn l2tp remote-access authentication mode 'radius'
set vpn l2tp remote-access authentication radius dae-server ip-address '127.0.0.1'
set vpn l2tp remote-access authentication radius dae-server port '1700'
set vpn l2tp remote-access authentication radius dae-server secret 'testing1234'
set vpn l2tp remote-access authentication radius server 192.0.2.5 key 'key123'
set vpn l2tp remote-access client-ip-pool start '203.0.113.10'
set vpn l2tp remote-access client-ip-pool stop '203.0.113.100'
set vpn l2tp remote-access client-ip-pool subnet '203.0.113.0/24'
set vpn l2tp remote-access ppp-options ipv6 'allow'
```