RBK23 satellite bad connection, continuously switching connections in network map (crosspost?)

Hi, thanks for your ideas!

>> What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?

I live in the Netherlands and the ISP is called Ziggo and the modem is this one: https://www.ziggo.nl/klantenservice/wifi/modem/connect-box, they call it Eurodocsis 3.0 modem.

The WiFi capabilities of the modem are turned off, otherwise default (automatic) settings. I can't tinker too much with it as the ISP is the owner of the modem. However I know the modem works and works well, and when connected through the modem's wifi (if turned on) or connected by cable I get great speeds. I also get good connection and speed from downstairs where the Orbi router is located (next to the modem).

>> What is the size of your home? Sq Ft?

Ground floor about 700 sq feet, middle floor 450, and top floor 450 sq ft.

>> What is the distance between the router and satellite(s)? 30 feet or more is recommended in between RBR and RBS to begin with depending upon building materials when wirelessly connected. 

https://kb.netgear.com/31029/Where-should-I-place-my-Orbi-satellite

My house looks most like the midsize multistory one. It's wood structure, brick and mortar, and plasterboards and so on (80 year old house). The WiFi signal has to go through the floors, which are plaster boards, empty space, then wooden construction and wooden floors. The distance from router to middle floor sattelite is about 18 feet, and distance from middle floor sattelite to top floor sattelite is also about 20 feet, maybe 25. Do you think the distance from router to sattelite on middle floor is actually *too small*?

>> Any Wifi Neighbors near by? If so, how many?

Yes, quite a number nearby but not on my channel (#1), or their signal is really bad. (I've also tried different channels but haven't really noticed an change). My direct neighbors are fixed at 6 and 11 (at my insistence, to rule out that explanation).

I just tested and I get only 25 Mbit/s download speed on the top floor (using https://www.speedtest.net) and that's without anyone else in the house using bandwidth.

FYI I didn't understand why the router was connecting to Sat-1 (on top floor) even though Sat-2 (middle floor) was closer. As if Sat-2 is perhaps bad ouf of the box? So I've now switched the sattelites: I've rebooted the modem and then turned on router, then turn on Sat-1 on middle floor (connects, no problem/magenta/purple color, but is just normal), then turn on Sat-2 on top floor (connects, no problem). So in this new situation, Sat-1 is on middle floor and Sat-2 is on top floor. Now I see that Router is connected to Sat-1 with 2.4G and Poor backhaul status, whereas Sat-2 is connected to Sat-1 with Good backhaul status. So the router keeps connected to the Sat-1 one (but now I expected it too, because Sat-1 is now on the middle floor) but it's still a poor connection. I don't understand why, because it's not badly placed -- should I put Sat-1 in a different room and see if it helps?

>> One User Experience/Configuration: https://community.netgear.com/t5/Orbi/Most-Stable-Orbi-Configuration/m-p/1941087/highlight/true#M970...

I've followed the instructions here but settings options on and off I don't see much of effect at all.