Not enough information here. Did you use this speaker earlier? Was the speaker used on S1 or S2 before? By someone else perhaps?

@106rallye asks some good questions.  If the Play:5 2nd Gen was used on an S2 system, you will need to add it to S2, then downgrade it to S1.  It’s a good idea to turn off all S1 devices while you do this, then turn them back on once the device is downgraded, then add it to S1.  

https://support.sonos.com/en-us/article/downgrade-a-sonos-product-from-s2-to-s1 

The Play 5 Gen1 used to work on S1 app and never on S2 app. 

I also have 2x Play3 Gen1 that were upgraded to S2 app.  As you will know the Play5 could never be added to S2app.

So I now have a Play5 Gen1 with a white flashing light, help!

Not sure what you’re asking here. Your PLAY:5 gen 1 should still connect using the S1 app. If you’ve updated the two PLAY:3s to S2, then they should connect using the S2 app. There is no communication between the two operating systems, though.

You may want to call Sonos Support directly to discuss it.

Flashing white, maybe try a temporary Ethernet to your router?

Flashing white

The Sonos product is booting up after being plugged into power. The status light will also flash white while it’s waiting to connect to the network.