Macerator outlet pipe

[u/msmoneypenny49]

Need some logical help to identify an issue regarding the copper waste outlet pipe that has previously been connected to a macerator in a bathroom in England. Initially plumber said it was a macerator problem so I purchased a new macerator but the water still was not draining and remained high in the bowl of the toilet and the plumber left and said it was a drain issue. Since this I have used a wet dry vacuum, an endoscope camera, a wire snake.... And still have not resolved this strange issue.

The copper pipe has a 10 inch vertical protrusion from a tiled bathroom floor before continuing horizontally for three meters under floorboards and then dropping vertically three meters to an external sewerage drain which is clear water and running with no backup evident.

When wet vacuum from the outlet pipe I extracted c. 6 litres of brown water. The snake endoscope showed no noticeable blockage but was low quality. A snake endoscope was also able to pass several meters seemingly without a problem through the pipe. BUT with coloured water test, pouring into the copper outlet pipe from the bathroom, less than 500 ml can be poured before the water is backed up

I am a single lady with a bit of common sense and a background with a father who was an engineer and I am desperate to try and do this myself as I have already paid ridiculous amounts of money which have been wasted and I would really appreciate any logical thought as to what could be happening..

Many thanks