Yes, if there is a different router with same SSID, either change the name or change channel, 1 or 6.
Turn that off for now and test with the 857.
If you need a DAP in AP mode you can use it however it needs to be on a different channel, 1 while the 857 is on 11.
Lets try that first. There is more resent FW available however lets test first...