DHCP Client IP Probl


I currently encounter the following issue which I have no idea on fixing. As a disclaimer I am by no means an expert and just struggling my way through this.
I run a OpenWRT VM on a Proxmox host. I have a Intel NIC card with 4 plugs that is given to the vm with pci pass through. One plug is configured as wan the other three are included in the lan bridge. I want to run this setup behind a Vodafone cable modem in bridge mode. I have not done any special settings on Openwrt.
Now to my issue:
if I have the isp box in router modus, openwrt works great, everything as I want it to. Internet access and all.
If I set the isp box in modem or bridge mode, openwrt gets an ip and gateway but I do not have internet access on any device on my network. I can also not ping anything on the internet from openwrt itself.
BUT if I have my pc directly connected to the isp box in modem or bridge mode that pc does get internet access.
This makes me think that the issue is with openwrt, but I honestly do not have any clue on where the issue resides as it works perfectly when behind a router.

I would appreciate it a lot if anyone could help me or give me a clue on what it could be here.

And I apologize for formatting and writing, this has been done on mobile.

  • Does the network on the ISP box and OpenWrt both match (If so, renumber one of the networks.)
  • Do you reset everything after re-arranging?
  • When connected with the OpenWrt, provide results of a traceroute to e.g.