I am still using Airport Extremes and they have always worked great for my house. Multi-story home with ethernet, but I have 0 issues with them. Easily getting 500Mbps+ with my iphone and more with computer/apple tv. Perhaps you just had something wrong with your setup, or maybe it was forced onto 2.4GHz?
I think its more likely my airports were just dying from heavy use. They were definitely not configured wrong. I would notice my iPhone X wanted to hang on to the 2.4GHz band while my iPad and laptop and even the wife's iHone 8 would be on the 5GHz band. I could never figure out why it wouldn't stay connected to the 5GHz band, as it would connect there in the same room but the moment I left the room it changed to the 2.4Ghz band. Oh well...lit is working much better now. I used the same network settings on the Orbi so all my devices would just re-connect, so I cane even rule out something fishy in my phone's network cache data about that WiFi as well.
Its worth noting I used to have much better speeds with the airports, this was a new thing that started to happen bout a year ago. I really think the radios were just giving out at this point.
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I’ve used the RBK852 and I’m now using RBK752 and if you’re using a wired backhaul to the router the backhaul is limited to 1000 mbps while the wireless backhaul connects at 2400 mbps, I can see the backhaul channel on a couple of programs I run and it consistently runs at 2200 mbps or so. Try it, you might be surprised, or course YMMV.
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I can try, the backhaul signal between the 2 is fairly weak, as the satellite is up a floor on the opposite end of the house. Not sure on the Orbi, but other mesh systems will use the extra backhaul band for the wireless clients when they are connected via ethernet.