Thank you for your help.
My router is a ZTE F680,
Hardware Version V4.0
Software Version ZTEGF6804P1T18
The router is giving me some trouble, because is quite fast, but I am experiencing some problems like absenceof NAT loopback or traversal Nat, thats why I am thinking about getting a neutral router and setting up this like a bridge.
Anyway, there is no indication in the router about L3 or L2 nor couldn't find anything related to this in the internet.
This is a FTTH router.
I tried your suggestion of it serve IP addresses via DHCP from a range that does not overlap with Pi-hole's DHCP range, but as expected only the router assigns IPS, and if I disconecct the DHCP server in the routerm well, this is the same I did at the very beginning and I don't get any IP.
There is no option to disable VLAN in the router as well.
I realized that there is a firewall in the router, I tried to disable it, but the problem persists.
Well I have investigated a little bit further and It appears thaa tht router is blocking the dhcp traffic, so I don't think that can be solved.
Thanks anyway