@JimBon
Your Internet connection went down, or you turned off ISP router, or connection between Main Deco and ISP Router is lost (Ethernet cable unplugged, bad cable?). You thought it is Deco issue, and rebooted Main Deco.
After reboot, Main Deco still can't establish Internet connection. This is when it starts giving IP addresses to devices in 192.168.68.* range. When Internet is back, Main Deco will switch back to AP mode and start getting IP addresses from your ISP router, in your 192.168.253.* IP range.
The home network should recover on its own, after Internet is back, because Main Deco sets very short DHCP lease time, under 2 minutes. Devices that got IP address in 192.168.68.* range from Main Deco will keep asking for IP address every minute.
Unless you have devices on your home network that don't properly support DHCP protocol - then, escalate to device manufacturer.
Next time Deco LED goes red on all Decos in your household:
1. Do not reboot (power off/on) Main Deco. It'll not switch to 192.168.68.* IP range if it stays on during the Internet downtime.
2. Verify that Internet is down, for example by accessing ISP modem/router management page and checking there. If router claims Internet is up, perhaps something is wrong with Ethernet cable between router and Main Deco - replace it.
3. Wait till Internet is back, meaning Main Deco can run successful Internet connection health checks. This is when LED will switch to green on all Decos.
4. Home network should recover on its own after Internet is back.
As an extra step to prevent large impact to your home network when Internet briefly goes down, set DHCP lease in ISP router to reasonably long time: at least few hours.
Not everyone is happy with Main Deco becoming DHCP server when it believes Internet is down. TP-Link promised configuration setting in firmware 1.6 to turn on/off that feature. Till then, understanding how Main Deco functions when its LED goes red will help you manage the issue.