IPV6 Cascade LAN to WAN?

I have 2 routers. One from the ISP (it connects directly with fiber optic) and a second one TL-WR840N. The ISP router is a custom one(Digi) - has only 1 lan out and I have almost no control in the settings. I connected a second router through LAN to WAN, my first router is and my second is My first router connect with PPPoE and the second Router is set on Dynamic IP (auto). Ipv4 works without issues but when I enable IPV6 the only way I can get an address is if I set the IPV6 config to Dynamic IPV6 and SLAAC. In my PC it shows no internet connection while on my second router I get the address and the status says Connected. There are other options as Static, PPPoE, 6to2 and the address type DHCPv6 and SLAAC. Before the cascade I had IPV6 internet connection. Could someone please help me how to fix it? or is it even possible to do this? I need the second Router because I have a Pi-Hole set up and not being able to change the IPV6 DNS its almost useless but now I have control of IPV6 but can't get it to work. Any help is greatly appreciated! The PC gets IPV6 addresses and default gateway but no internet access on IPV6.