Undo all changes to get back to working configuration: ISP modem in router mode and Deco mesh in AP mode.

Then, proceed with the following order:

1. Configure Deco mesh to run in Router mode. Verify you have Internet and Deco mesh is healthy (online).

2. Change ISP modem to bridge. Power off and on Main Deco.

There is a big chance everything will just work if done in that order.

If not, if after that you do not have Internet connection - contact your ISP tech support with the question of how to configure third party router (Main Deco in your case) to work with their modem which is in bridge mode.