SSRS report datasets

Hello everyone, I need help with an SSRS report.

In my matrix, I need to use columns from two datasets. Initially, I created two datasets and used the lookup function to add the desired columns from the 2nd table. The filters were working fine, but I noticed the rendered content is not correct compared with the power report. I switched to another approach creating one big dataset combining columns from two tables using an inner join. As a result, I got this error "The Group expression for the grouping 'Rig' refers to the field 'Rig'. Report item expressions can only refer to fields within the current dataset scope or if inside an aggregate, the specified dataset scope. Letters in the names of fields must use the correct case.".

If someone can help me I will appreciate it very much! I am still new to SSRS and don't know which approach is better.