I have access to our partner sandbox in DTM which works fine when I select partner sandbox as company. I have provided all rights to my Adobe ID for a different company, but when i select that company, i don't see any web properties for that company. I just see partner sandbox property, that too disabled.
Are you able to log into DTM? If so, what are you seeing? I'm wondering if you are still having some sort of permissions issue or if the web property in question is part of a different DTM company. Any screenshots you have would be helpful.