Here's a screenshot of my history via the app running on my phone, as well as a speed test on my AT&T Gateway: @IrvS

My 1Gig service was getting expected wifi speeds when I did a speed test on 12/5 (and every test prior). When I did another one 2/1 (and every one since) my speeds have dropped considerably, like...

The only change I think I made in that time period was I changed all networks except my guest network to the same name and password to let each individual device connect to the best network per it's...

Should the Deco unit connected to the AT&T Gateway have a different Internet Connection Type than the other satellite Deco units? I currently have them all, for both IPv4 and IPv4 set to "Dynamic IP".