I appears you have done your homework, so here is something else to consider. Could some large metal object (say, a refrigerator) be too close to either the router or the Dot so that it could be interfering with, or blocking the wi-fi signal?
Do you own your router or is it leased from ISP?
What is brand and model? I presume it is a dual band router?
Even though you put the Dot on 5G, it could have jumped back to 2.4G (which channel 11 is). Do you know what channll 5G uses?) Reason I say Dot may have jumped back to 2.4G is because you have set Dot to both 2.4G and 5G in the past,and since we cannot "forget" one of the bands via Dot, if something happens to cause one band to quit, even for a short while, the Dot will jump to other band if available.