Is the EC and CRC from a DrayTek modem attached to the ASUS RT-AC86U, I forget what you have 
I cannot see why the Modem would be effected by the recent upgrade of your RT-AC86U, Maybe worth just switching both the modem and router off for 15 minutes and then bringing back online and see if that settles down the modem etc.?
But this is most odd ?
In a completely sane world, madness is the only freedom (J.G.Ballard).
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