If neither the bath nor the basin won't drain, then that's pretty indicative that there's a restriction on that last bit of waste run after the bath tee.
I'd cut the waste run back to the single pipe heading out the wall and then use compression fittings and a bit of pipe and use that to check if that pipe is blocked.
I'd cut the waste run back to the single pipe heading out the wall and then use compression fittings and a bit of pipe and use that to check if that pipe is blocked.
