@unmesh While I am inexperienced with trying to separate the two WiFi channels to different subnets, I noticed that with my RE605X, RE815XE, and RE235BE, that they do have their own DHCP server, AND the default setting is "AUTO".  Maybe making sure this is set to Off under -> More -> Advanced Settings -> DHCP Settings will help resolve your concern...

  @jzchen 

I simplified my setup by using just the 2.4GHz band for the client facing and the Internet facing connections and I turned off the DHCP server.

Clients are now not getting IP addresses over DHCP from the upstream DHCP server. On the other hand, if I set up IP addresses manually, they can access the Internet fine.

unmesh wrote

On a related note, does the RE315 uplink to both WLAN bands or only to the one it deems best, treating the other one as a backup?

I have an RE215 and have the same question.  In auto mode, where it's connecting to both 2.4ghz and 5ghz bands, and also extending both bands, does it just pick one of the bands to uplink on, or does it uplink 2.4ghz traffic on 2.4ghz and 5ghz traffic on 5ghz?

I can see the RE215 connected to the AX1500 access point on 5ghz, but I'm not sure if it would show it connected to both bands.  Is everything uplinked on 5ghz, then?

  @david3 Since each band can have a separate SSID it extends  2.4 <-> 2.4, 5 <-> 5 and if tri-band 6 <-> 6.

For what it's worth, I've got the same SSID set everywhere, for 2.4ghz and 5ghz on the main AP and the extender.  My phone seems to roam OK with minimal disruption.

  @david3 

That is great to hear! What is your upstream AP?

I'm using Rcukus Unleashed which may be the problem.

  @unmesh My upstream AP is a TP-Link AX1500 in AP Mode.  I use a separate Mikrotik router.

I guess I spoke to soon.  I'm finding, if my phone switches between bands connected to the extender, I have no network connectivity, even though the wifi is connected.  Both uplinks are green with 3 bars.

DHCP is handled by the router, not the AP or extender.

If I use Hi-Speed mode and uplink on 5ghz and repeat on 2.4ghz, it seems to work fine.  It's too bad it has issues extending both bands.

  @david3 

My set up is similar but with different network elements: Extender to Ruckus APs to pfSense router/firewall which has the DHCP server.

I repeated the setup using only 2.4GHz bands and turning off the DHCP server in the Extender. Now the iPhone is getting an IP address from the DHCP server but the Windows laptop still isn't. If I set up an IP address manually on the laptop, it has full network access.

Weird!

  @unmesh There is/was a command line command set:

ipconfig /release

ipconfig /renew