Getting an error in R7000 "The IP address conflicts with the WAN IP subnet".

Thanks for the thorough answers. I've replied to them below, but the conclusion in my second post is going to be the solution I'm going with, at least until I upgrade hardware. I've had too many issues with Netgear and DHCP over the years, and with others echoing the sentiment that Netgear has a bad rep for DHCP (can't remember, but I think that was some answers on Quora the last time I had DHCP issues with this router, before running my own DNS), I'll just stay clear of letting the Netgear hardware run my DHCP.

> When I try to make (aaply) changes in DHCP, [...]

   What are these "changes"?

<< Reserving an IP for a device, as I have done for ~100 before.

> o I have assigned dozens of static IPs, [...]

   Terminology: A "static" address is configured on the device itself.

What you configure on a (DHCP server on a) router is a reserved dynamic
address, not a static address.  Either one should fix the address of a
device, but some implications are different.

<< OK, I'll take note of that.

> [...] but this has been put in bridge mode, [...]

   Your Screenshot 2020-12-15 at 22.38.26.png seems to agree with that,
but if you get that "IP address conflicts" message, then I worry that
the Internet Port : IP Address has changed since the time when that
picture was taken.

<< No, the picture was taken after the issues started (and was the same a couple of years ago when I initially set it up.

> o o However, the Sagemcom does still broadcast a WiFi. [...]

   I would not expect that from a modem-only, but your experience with
that device exceeds mine.

<< It's a buggy as hell modem/router, but it's what my ISP provides, and they say that a lot of third party modems will not work. SO I've been using it out of necessity.

> o I have a DNS server locally (192.168.0.66), [...]

   You might try without that.  Users have reported problems on some
Netgear router models/firmware versions when using DNS servers on the
LAN.

<< I did, see my next post. It no longer throws an error when I try to "apply", but it still doesn't give the correct IP.

> o I have a Nighthawk X4 AC2200 connected via ethernet, set up as AC.

   "AC"?  Wireless Access Point?  Shouldn't matter, but a simple
configuration is usually easier to diagnose than a complex one.  I'd
switch it off for now.

<< Yes, AP (typo).

> [...] and there's no new firmware (see attached screenshot).

   Says who?

   Visit http://netgear.com/support , put in your model number, and look
for Downloads.  (For older versions, under Firmware and Software
Downloads, look for "View Previous Versions".)  Find the kit(s).
Download the kit(s) you want.  Read the "Release Notes" file for
instructions.  (In the User Manual, look for "firmware", and,
especially, for a topic like "Manually Upload Firmware to the Router".)
When that fails because of a deficient User Manual, try:

      https://kb.netgear.com/23960
      https://kb.netgear.com/31573

   A Web or forum search for that version should find a few complaints,
as well as many suggestions to try V1.0.9.42_10.2.44.

<< Well, I tried upgrading from router settings web UI, so the R7000 is "Says who" :)

It says "No new firmware version available." when trying to update (tried again today, same message). I'll look into a manual update, since the router UI can't find it.

> If disabling the PiHole DNS, the error no longer appears. [...]

   That was: "I have a DNS server locally (192.168.0.66), [...]"?

   I'd try different firmware.  One of the newer ones might have solved
this problem.

<< Yes, but as noted, I've moved all my IP assignments to the PiHole, and everything works fine now. Even if a new firmware would fix the issues, I don't think there's an easy way to import 150 IP assignments in bulk in the R7000 (there is in the PiHole, which is why I tried), and I'm not going to manually input them all in the R7000 again :)

> If disabling the PiHole DNS, the error no longer appears. [...]

  PiHole issues crop up here pretty regularly. Did you do the usual community search?

<< I didn't, since I only found out that it could be related to the PiHole after my post. But even then I didn't, because using a different DNS still didn't allow assigning IP reservations, it only stopped the error messages. So even with no PiHole, it couldn't assign the correct IPs.