Short: I think I need to switch the main Deco to AP mode, but I welcome any corrections, insights or additional info

Setup: Have Starlink (gen 2 w/ethernet adapter) and local ISP ADSL line (rj45 coming out of ISP modem) connected to an Omada router (ER605); the Starlink is primary WAN, ADSL is setup for Line Backup.

Then I have three Deco x50s with ethernet backhaul between them, the Main Deco (which has been in Router mode) was connected to the Omada Lan port. 

The other two Deco x50s are connected (sequentially) via ethernet, for ethernet backhaul.

Tangential context:  My house has very thick stone walls, the Starlink generally works well, but seems to have issues with concurrent video calls as wells as the kids' favourite streaming service; the ADSL is fairly consistent but rather slow speeds (abysmal upload speeds). For those reasons, I have gone with the above setup in hopes of ensuring decent and constant internet throughout the house.
I am not very knowledgable when it comes to IT issues, I read the internet and lack proper background understanding which leads to mistakes.

Issue & Question: Currently (I have only had the Omada installed a few days) the network (the x50 mesh network)  has frequent DNS issues and other unidentified failures (unidentified by me at least, but the Deco app says no internet).
I have been using the main Deco (connected to the Omada) in router mode, but maybe I should switch it to AP mode?  Or is there something else? Do I need to setup VLANs? Do something with DNS settings?