Dear @saintofinternet ,

saintofinternet wrote

I am using 3 CPE510 Devices.

1 is configured as an AP

Other 2 are configured in Clients.

When one client is plugged into an Ethernet switch the AP-Client wireless link works flawlessly.

As a backup when I plug the other Client into the same Ethernet switch the network becomes unstable and the packets start to drop.

Why is this happening??

Look carefully at the topology diagram you provided, there is indeed a problem, if you connect two client CPEs to the switch at the same time, this will cause a loop.
And you can't achieve CPE link backup this way.

Best Regards!

Dear @saintofinternet,

saintofinternet wrote

How can I achieve that with the hardware I have?

You can consider get two managed switch which can support STP function(STP Security prevents the loops caused by wrong configurations or BPDU attacks. )

The network topology like: Switch --CPE AP )))((( CPE Client --Switch.

Best Regards!