We changed from Mixed all modes to a single mode operation. In some cases this helps the single as the radio doesn't have to do multiple modes any more, just one so the signal is improved.
Also be aware that the 857 has better WiFi connections speed than the 1525, the 857 supports up to 450Mb connections on 5Ghz, were the 1525 only supports 300Mb.
Keep the 1525 on hand for now. You could it it as a bridge later on.