If you can't find the link between the two families and don't have DNA matches then it could be that they are not closely related if at all.
For years I puzzled over a newspaper obituary which went into great detail about the funeral of our deceased relative- place, mourners & their relationship, wreaths, etc. With an already extensive family tree it was easy to identify each mourner and confirm their relationship- except for one named individual. This person (named Robert) even had his residence listed. The problem was not being unable to identify him but to figure out how Robert was or even could be related. Now, the deceased had moved with his family to their nearest town when young. Robert also lived there but he and his family were also easy to trace. Newspaper snippets were very helpful and through them I was able to establish that Robert was not related in any way. Seems the reporter was able to identify him at the funeral, knew both men were business men with links to the same town and called then cousins!