1- Have someone snake the line UPSTREAM and down stream from the main trap ( min 3/4″ wire with large head 3″ hread for a 4″ line)
2- Remove the toilet and snake the line from this drain also
3- While toilet is off the floor flange have someone check the toilet horn (outlet) for possible obstructions Q TIPS etc
4- while toilet is off throw 30 sheets or more of toilet paper down this drain pipe and dump a 5 gallon pail of water while someone is looking at the house trap to see the paper passes through
If all goes well and you get a stoppage again call for a video inspection with a locator to pin point the exact location of the sewer defect (back pitch or split joint etc)
GOOD LUCK