@roytanck 

Barring a bug in the Deco firmware, the LAN IP should be persistent.  It is on mine.  However from other discussions I understand that the Deco will pick a new LAN IP if it detects another device using that IP during bootup.  This includes it being in use on the external network (for example on the cable modem or similarly where you are trying to wrap an existing router to use as a modem)

I think this is a poor design choice as the one thing you don’t want moving IP is the router - everything else should move!  Far better if it reported the conflict rather than this attempt at a silent fix.  

That aside do look at your network to see if there’s something conflicting.  

@ajharvey Thank you. Most of my reboots are because of firmware upgrades, so it could be that the LAN IP reset only happens with new firmwares.

There are no conflicts on my LAN. The modem is in bridge mode and in another range, and there's just one DHCP server (the Deco). Usually, the LAN IP is reset to 192.168.0.x.

I agree that the router IP should never change unexpectedly, and I'm a little disappointed by how the app handles this. The steps to fix this when it happens currently are something like this:

* Tell Android to stay connected to the wifi, even though there's no internet.

* Disable mobile data on my phone.

* Open the app, and wait a long time for it to connect to the Deco (usually takes a few attempts).

* Temporarily set a public DNS server (i.e. 1.1.1.1) under advanced settings.

* Set the LAN IP to the original (desired) setting (sometimes also takes a few tries).

* Restore the DNS setting to use my local DNS proxy.

* Re-enable mobile data.

This takes ten minutes or so because the app takes a very long time to connect to the local router, even when local wifi is fine.

I'll see if I can try a simple non-firmware-related reboot tonight, to see if the setting persists.

Quick follow-up: I rebooted my M5, and the LAN IP was reset to 192.168.0.1. So, the problem does not just happen when the firmware is updated.

At this point I suggest you go to the web interface (on the IP address of the main Deco) and go to Advanced -> System -> System log.  Read through the entries from around the time of the reboot and see what the Deco thinks is going on.  Note that you can filter the log entries ( DEBUG is everything as far as I can tell). If nothing obvious presents itself you could save the log and post a section of it from the time around a reboot here.  

The log will be of use if you need to log a tech support case anyway.