I installed PiHole on a Raspberry Pi using a container, about a couple of years ago, and have struggled intermittently to get it working with my Shibby Tomato router.
I've come across various suggestions on the 'net, with no success - there appears to be no definitive guide, with users just basically muddling through. I've always ended up restoring a previous configuration on the router and using the built in Adblock.
This post is a last whimper before I abandon the idea altogether - I'm 81 years old and I don't want to spend my remaining years in a futile struggle with PiHole!
Pi-hole is very flexible and nothing in your description seems to indicate we can do anything it ourselves. When you don't find any users reporting success here, it may just mean nobody had success with this router (assuming it is too much locked down to change DNS settings?). Without knowing what is not working, it's also difficult to give suggestions.
I see there was an older discussion with some unanswered questions at the end:
We could revive this if the problem is still the same.