@DarrenRobert 

Not sure what went wrong, but if you want to set up Deco mesh you'll need to start again from the beginning.

1. If you see one or more Deco M9s in Deco app, remove them in app and reset them

2. Undo all configuration changes you've made on CenturyLink C4000 modem/router, such as "putting the modem into transparent bridge mode". Confirm you have got Internet access back, through C4000.

3. Connect first Deco unit you are going to configure to one of CenturyLink C4000 LAN Ethernet ports, with short good quality Ethernet cable. It'll be your Main Deco. Ideally, do that with Deco unit you haven't used before - just to start fresh. 

4. Configure that Deco through Deco app

DO NOT reconfigure your CenturyLink C4000 router if you can't complete Main Deco M9 initial setup again. That won't help, but could just hinder the troubleshooting. Deco M9 will work just fine, for the purposes of initial setup and configuration, when connected by Ethernet cable to working ISP router. 

If Main Deco still fails on Internet connection test, check Device Table in your C4000 under Modem Status. Do you see Deco listed there? Could be unknown device name but with Deco MAC address you can find on a sticker at the bottom of Deco unit you are configuring.

A) You don't see Deco in Device Table - that means it can't even connect to CenturyLink C4000;

B) You do see Deco in Device Table - it means it has physical connection to CenturyLink C4000, but can't verify Internet connectivity by pinging Web sites or sending DNS queries.

If A - possibly, Deco unit hardware issues, problematic Ethernet cable. Also worth trying connecting Deco to a different LAN port of CenturyLink C4000.

If B - you may have changed default content filtering settings on CenturyLink C4000, such as enabling whitelist (if this feature is present) or setting Service Blocking. Review and undo these changes.

Finally, just curious: if I have found correct specs for CenturyLink C4000 - it is quite nice and powerful WiFi 6 router. Why do you need to replace it with WiFi 5 Deco M9 instead?

Don't get me wrong, my Deco mesh runs on M9s and I am quite happy with it, but my router is WiFi 5 and really sucks when it comes to WiFi.