There are no know issues with joins in wappler.
The process is simple:
Open the query manager
Select your first table:
Select your second table and a join dialogue will open:
Select the foreign key field from that table:
Select the matching primary key from the master table:
You have an option to alias the second table (really useful for self joins)
the resultant SQL can be seen at the bottom of the query dialogue
I have created multiple joins on several occasions (10-15 joins) without any problems
If your query is not working perhaps you could post details of it so we can make a more informed recommendation