Mine are 68U and 86U and BOTH got same 2,4G problem as you.
Sometimes after restart my card I get up to 100 or even 130Mb/s, and next measurement I have 1Mb/s.
Strong signal >-30dBm.
Same on PC an mobile S7.
Stock and Merlin ...
I really cannot understand whats going on here!
First moment back to legacy Merling 380.70 and all seams to be fine with 100Mb/s on my 68U.
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10 minutes later nothing changed I have <2Mb/s again ...
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Ping still ok, upload not good but working, download terrible!
How can it be no issue for most others here and I have it on both units???
Nobody looking how 2,4G is performing?
I think it has to do with 40MHz bandwith setting in 2,4G, but less is no option in my case with no neighbours!!!