Interested in this post as you upgraded from the 2862ac to the 2927 which I had planned to do. I found however that the 2862ac gave me 950/50 with my Virgin Gig1 service, even with firewall enabled. What speed were you getting with your 2862ac that persuaded you to upgrade ?

I changed from a 2862AC to a 2927AC because I moved to a FTTP connection with 500Mbps up and down and the 2862AC was, from what I had read in various places, only meant to support 300Mbps. Also as I no longer needed the ADSL/VDSL connection the 2927AC seemed the best model to change to.

With the 2862AC I was only getting around 300Mbps download when running speed tests, which seemed to confirm what I had read about 300Mbps being it's maximum speed. However when I changed to the 2927AC the speed tests only gave around 350Mbps. This was using a Windows 10 pc connected directly to the router with a network cable.

What I noticed was that testing with different speed testers I was now getting significantly different results. Before changing to FTTP I was on a 16Mbps up and down connection and had run speed tests using speedtest.net, speedof.me, nperf.com and broadbandspeedchecker.co.uk and all of them gave similar speed results. Now on the FTTP 500Mbps up and down connection broadbandspeedchecker.co.uk was consistently giving around 350Mbps up and down. All of the others were giving completely different and varying speeds ie speedtest.net gave a maximum of around 120Mbps up and down. Speedof.me gave around 250Mbps down and around 150Mbps down and nperf also gave different figures. So I suspected my broadband supplier was not giving me the expected 500Mbps up and down.

On my Windows 10 pc I run a copy of Linux Mint in a Vmware virtual pc and thought I would run a speed test using that, expecting it to be similar or slower as it was a virtual pc, but using broadbandspeedchecker.co.uk it gave around 500Mbps up around 350Mbps down. Running tests using the other speed testers also gave much faster speed results but none were as fast or as consistent as using broadbandspeedchecker.co.uk. So it seems that something in my Windows 10 setup is causing the downloads speeds to be reduced. When I get time I'll try to find out what it causing it.

After reading your post about getting 950Mbps down using your 2862AC I temporarily swapped back to my old 2862AC and ran a speed test using broadbandspeedchecker.co.uk in the Mint virtual pc and that also gave around 500Mbps down and 350Mbps up with Hardware Acceleration on, the firewall enabled but the Data Flow Monitor off. If the Data Flow Monitor was turned on the speeds dropped to around 300Mbps up and down.

If I had known that the 2862AC was capable of giving 500Mbps, or in your case 950Mbps, speeds then I wouldn't have bothered changing to the 2927AC!

I realise I'm very late to the party here, but thought I'd post this in case it helps anyone in the future.

I am seeing the same issue as the OP.
ie I try to enable WAN3 (2.4G WAN) to act as a WAN backup to my iPhone hotspot should the main WAN1 connection drop.

I also see WAN3 and WAN4 disappear from the list. (It now shows only WAN1, WAN2, WAN5 & WAN6).

However it was actually connected to my phone hotspot and my LAN clients were able to access the internet through the 2927+iPhone.... Eventually...

Note that there were a few issues along the way...
Firstly my iPhone is normally set to connect to the 2927's built in WiFi LANs (and also a separate WiFi AP point I have).
For the hotspot on the iPhone to kick into action though it must not be connected to WiFi, it must be connected to 3G/4G/5G.
However if you switch the WiFi off on the 2927 (to disconnect the iPhone from the LAN) then you can't use the 2927 to connect to a hotspot acting as a WAN.
To get round this you can set your phone to "forget" the local WiFi networks, and leave the WiFi enabled on the 2927. The iPhone then connects to 3G/4G/5G and can act as a hotspot.

After fiddling with the above and getting a connection through the hotspot I then wanted to set WAN3 as a Backup for if WAN1 failed.
This should be possible with WAN > General Setup > WAN 3 > Active Mode
However if you change Active Mode from "Always On" to "Backup when WAN 1 fails to connect" it throws an error message: "WAN1 is already for backup!"
I think there's a bug in there, it's as if WAN1 and WAN3 are actually presenting as different ways of accessing the same WAN.
It did seem that if WAN3 was set to "Always on" and both WAN1 and WAN3 were connected at the same time, then the active route would be WAN1 with data failing over to WAN3 if WAN1 disappeared.

As for the disappearing WAN3 and WAN4 in the menus on the portal, I found they came back if logging off and on the portal rather than rebooting the router.

So in summary I could probably just about use a mobile phone hotspot as a backup WAN if pushed, but it's all a bit fiddly and buggy at the moment.
It feels like it's needs some bug fixes and a clear setup walk through guide.