BTW. Next time you need plumbing work done, it could be worthwhile putting in an inspection opening at the boundary, which is known as the ‘tie point’. Sometimes the pipes have provision for one but without a capped riser to the surface. If there is a blockage, you can shine a light down the riser and immediately see if the problem is within your property, in which case you call a plumber, or outside your property, in which case you call the utility provider to get them to fix the problem. Like you, they will be able to see unequivocally whether the problem is yours or theirs.
You tell the difference by whether you can see water in the riser. If there is water, the blockage must be below the tie point and in the utility provider’s area of responsibility. If there is no water in the riser, the blockage must be above the tie point and in your area of responsibility.