One thing to bear in mind with the design planner is that it assumes that you’re going to have the APs on maximum Tx power.
To be fair I’m coming at this from a high density deployment frame of mind because that’s what causes the most issues, in a house one up and one down on high Tx power might just be ok, but depending on the make up of your walls and the size of your gaff you’ll probably not get 6Ghz rates outside of the room of the AP.
Also have you changed the radio on the bottom right little “red-amber-green” scale? By default it’s set to 2.4Ghz, also on this scale you can click the actual dots and tweak the “power levels” for lack of a better word.
I’d put the green dot to -59dBm and the second dot at -70dBm.