Clogged Drain line

I just bought a house that was flipped by a contrctor that knew little to nothing on fixing up a home. that being said:

I have a clogged sewer line from the master toilet. I have tried a commercal grade cleaner, no result! I have snaked the drain up to the clog but after a number of tries the snake will not penatrate through the clog. I believe the clog is below the junction where the sink ties in. I think this because if you run the sink for a period of time, water backs up in the toilet.

I see under the house, on the drain line there is a cap that I can unscrew so I can access the line. By doing this I believe I can remove the clog or snake. Is the sewer line filled with water or is it dry? Is unscrewing the cap an OK idea. Also a side note the lines are cast iron, which I know clog eaiser. Any suggestions/help is welcome

Thanks for those reply