Version 3.0.0.4.386.46061 seems to be suitable for your RT-AX86S and was released ages ago, so if there was any issues – I would have thought they would have sorted it by now.
Most odd 
I will make some inquires as it is probable only suitable for RT-AX86U 
If manual update fails again, even after you have rebooted router, can you send feedback form with ‘UK Sentinel’ in comments box, so I can get them to chase feedback.
Let me know either way ?
In a completely sane world, madness is the only freedom (J.G.Ballard).
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