A relative of my neighbours used a device with like a coiled spring on it (powered) which he managed to get in about 6 foot down the pipe and then retract. This pulled out the debris and muck that was causing the blockage. I wasn't present at the time so can't describe it better than that. There was no jetting involved.
fwiw: the "simple coiled device" is called a "WC Auger" best in max lengths of about 2m (6ft).
they are great for getting around trap bends in WC's & gullies.
they have a short plastic sleeve to prevent WC scratching.
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