Yes, Fritz!Box are a good units and very reliable, I have 3490, alas I had a look at some other forums for the DSl-AC88U Annex B and it does seems as though 35b profile does not work well.
Most Friz!boxs support Mesh Wifi, so radio signal issues/strength can be resolved
Maybe worth sending ASUS a feedback form and asking them to provide guidance if you cannot return the DSL-AC88U and/or just sell as you say.
DSL-AC88U = 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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