We just bought our condo on the 4th floor few month ago. The toilet in the guest bathroom was clogged then and after few attempts to unclogg it we replace it with the new one. So we are 100% sure that the problem isn't in the toilet itself. After replacement, toilet get clogged very lightly several times, but few pushes with plunger helped. Today I realized that toilet is completely clogged and plunger doesn't help anymore. Water drains but veryy slow. Shower and toilet are close to each other and when toilet is clogged water ifrom shower doesn't drain either. I don't know how to deal with this problem. Toilet in the master bathroom works fine, but this one makes me feel bad. Can anyone please help me with advice? Our apartment was built in 1971 and as far as I know neighbors don't have this problem. Thanks in advance.

Remove the toilet and snake main drain through opening in the floor. You have clog somewhere under this bathroom where all pipes connect in one common pipe...

Remove the toilet and snake main drain through opening in the floor. Because it creates such a dirty mess to snake from a open toilet hole I pull a toilet as a last resort If I can't find oother way.
You have a clog downstream from the shower. Most bathrooms are plumbed like this, (see image) sending a snake from the lavatory roof vent will reach the clog. Put out enough snake to reach the base and 18 feet more. Good luck, Tom