No IPv6 when connecting via DHCP

As stated in the title, I can't get an IPv6 connection on the devices connected to OWRT when using DHCP. If I connect directly to my ISP's router, be it on bridge mode where I set my PC to dial up the PPPoE connection, or on router mode where the device dials the PPPoE connection itself, I can get IPv6 normally.

If I set my ISP's router on bridge mode and set OWRT to dial up PPPoE, I do get IPv6 working too, but since my throughput is heavily decreased doing that, I would rather have it connecting to the internet via DHCP.

Which info I need to provide in order to mitigate this?

Any help is appreciated.