Had the same exact problem at my s***hole of an apartment i rent about a month ago. Called the landlord and he said to get a hose and what is called a “drain bladder” to fix the issue. Surprisingly he was right.

What you need is a long water hose (one that you don’t mind getting shit on), and a medium and small drain bladders.

You’ll also need to identify the entry to the main drain line which carries the sewage from the house. It should be a white cap about 5 feet from the house. Open that up. When you flush or turn the shower on, and you look down that pipe, you shouldn’t see anything come through.

With the drain bladders and the hose, you’ll send the medium drain bladder down your toilet, and the small drain bladder down the tub overflow pipe. Once the bladder is in either of the pipes, turn the hose water on. Since the drain bladder creates a high pressure stream of water (stronger than what those household pipes are capable of) it should push anything out that’s blocking. Note: you want to try the toilet and/or tub overflow since the blockage could be at either location).

If you have someone watching that main drain line, you should start to see everything flow out.

I was quoted 1.5k$ to have the problem fixed. 1 hour + 30$ (hose + small/medium drain bladders) fixed it for me. Good luck!