Do you mean that as long there there is 1 device that doesn't support WiFi 6 connecting to the WiFi 6 router, I won't be able to get OFMDA for all devices connected to that router?
Essentially yes, because that one device does not understand how OFDMA works, so it falls back to a "lowest common denominator" which is OFDM (without multiple-access, ie "time slicing"), so to speak. Of course there may be moments that one device is silent so other devices will be able to be on OFMDA. But as long as that one device remains active on the network, it will force it down to OFDM cos that's a common standard everyone can agree on.
In lay speak, that one device will be screaming "Eh I don't understand what's going on with this gibberish" and essentially force everyone down.
https://www.5gtechnologyworld.com/ofdma-improves-spectrum-use-in-wi-fi-6/
The diagrams are quite clear.
To further elaborate, this is a diagram illustrating just a "pure" network environment without interference. For most of us who live in apartments (HDB, Condo), there's a lot of neighbour traffic. All those devices are using the same frequencies, so the "sharing" includes times to wait for your turn" because your neighbours' devices are all sharing the same wireless channels. This is what a heavily congested network can be. That's why it's a great practise to keep your wifi power to a minimum (just enough to cover the area needed and handover to the next AP effectively without loss of wireless coverage) and use a wired backhaul, rather than to boost at max or over the limits and be a "noisy neighbour" affecting everyone else. The wireless backhauls require some form of boosted power so that you can have strong signals for the interleaving APs.
That's why I find this push to go with AX mesh systems and all very premature. When you consider that the 802.11ax standard includes the 6Ghz specification (ie, WiFi6E), then it's pretty messed up that everyone is just wasting money buying equipment without much of the truly meaningful gains to be had imo. As long as their are legacy devices and all, it'll be quite messed up. Those who know about these segregate their wireless and network topologies with VLANs to maximise capability and minimise penalties. IoT devices and slow devices like wifi cameras are left on a separate network for good reason. Am waiting for IMDA to open up the 6Ghz spectrum properly for Wifi, and for the proliferation of Wifi 7 devices. The devices on the 6Ghz band are going to be OFMDA compatible since that band is part of the new standard. Kind of like how 802.11ac benefited more from the move to 5Ghz (even though 802.11n was on the 5Ghz band).
Of course, most users won't understand or care - and just throw everything into a single basket, thus negating all the benefits but bragging because Windows reports the MCS spec as some fantastic high number (which may not necessarily be the actual speeds you're communicating at).
OK. The 2 routers are connected via Cat 6 cable. So there's no disadvantage of having the faster/newer router as an AP rather than the main router?
In terms of wireless performance, probably negligible. However, in terms of processing power, the newer router could have a better/more capable CPU and chipset. You'll need to investigate to be certain. This is one of the things about consumer "all-in-one" routers that can be a bit confusing. An older "router" could have a more capable processor.
With the more advanced networking systems around today (eg Unifi, Omada, Aruba, Ruckus), a router is a router, an access point does AP duties, a Switch just does its switch duties. It is not the same CPU being tasked to do multiple tasks and more unnecessary things (Antivirus? File storage? Other totally unnecessary funny functions? ).
PS: To be clear, I'm not saying don't get it. I'm simply saying know what you're getting into, but don't expect AX to be very much of an improvement over AC - it's not. The improvements will come only when you move to the 6Ghz band with Wifi 6E or more likely Wifi 7, only because you can know for sure all devices on that band support all the improvements. From a practical perspective, I'm more in favour of getting equipment that gives good 300-400Mbps coverage all over the house with the 5Ghz band.
