I’m not understanding the flashing blue light? Here is a summary of the status LED colours and their meanings…

https://support.sonos.com/en-us/article/sonos-led-lights

If I factory reset my own Sub-Mini or Sub, from power on/button press (& hold), I just see an orange flashing light to begin with, at which point I let go the join button and a few seconds later it turns to flashing green and in the App they each popup for adding to the existing Sonos Household/Speakers, which I see available and previously setup in the App. In my case I use their 8 digit pin code to connect them as printed on the Sub (I always take a photo of the code prior to any setup).

If for any reason the popup does not show in the App to setup the Sub, or I accidentally dismiss it, I use the ‘Add Product’ feature in ‘Settings/System’ to start the install. 

Note: the controller device for setup must be a mobile device that meets these requirements:

https://support.sonos.com/en-us/article/sonos-app-requirements

Ensure there is no security software on the phone blocking the setup, such as VPN client, Firewall etc. especially if it’s a works phone.

Check that the Phone is connected to the correct WiFi network, maybe use the 2.4Ghz band and ensure the WiFi signal is 802.11b/g/n compatible with security mode WPA2 AES.

If you do wire the devices to the LAN, only do that to the main router only (whichever router you are using?) - if the Eero hubs are ‘bridged’ then switch off the WiFi adapters on the (ISP provided) 3rd-party router and ensure there is only one DHCP server operating across the network subnet and that it is enabled with IP addresses available and that any device MAC filtering is switched off.

Hope that assists.