I noticed cpe's (sxt lite5) change default ap which are useually connected to every day.
There are 4 AP's, each of them provides internet connection. They all got a good LoS (-60/65dbm) with every CPE.
I'd know the reason of this behavior, is due to changes about the signal strenght? During this change customers lose their internet connction?
Shoul I lock every CPE to a single AP?

Client should not switch AP, if the wlan signal don't "drop". So this indicate a problem between CPE and AP. You might try to change some settings to wlan interface to client, to se if you can get it to try to reconnect a bit more, before dropping connection.

Try to play around with this settings:

disconnect-timeout (time [0s..15s]; Default: 3s) This interval is measured from third sending failure on the lowest data rate. At this point 3 * (hw-retries + 1) frame transmits on the lowest data rate had failed. During disconnect-timeout packet transmission will be retried with on-fail-retry-time interval. If no frame can be transmitted successfully during diconnect-timeout, connection is closed, and this event is logged as "extensive data loss". Successful frame transmission resets this timer.

distance (integer | dynamic | indoors; Default: dynamic) How long to wait for confirmation of unicast frames before considering transmission unsuccessful. Value 'dynamic' causes AP to detect and use smallest timeout that works with all connected clients. Acknowledgements are not used in Nstreme protocol.

hw-retries (integer [0..15]; Default: 7) Number of times sending frame is retried without considering it a transmission failure. Data rate is decreased upon failure and frame is sent again. Three sequential failures on lowest supported rate suspend transmission to this destination for the duration of on-fail-retry-time. After that, frame is sent again. The frame is being retransmitted until transmission success, or until client is disconnected after disconnect-timeout. Frame can be discarded during this time if frame-lifetime is exceeded.

on-fail-retry-time (time [100ms..1s]; Default: 100ms) After third sending failure on the lowest data rate, wait for specified time interval before retrying.
periodic-calibration (default | disabled | enabled; Default: default) Setting default enables periodic calibration if info default-periodic-calibration property is enabled. Value of that property depends on the type of wireless card. This property is only effective for cards based on Atheros chipset.
periodic-calibration-interval (integer [1..10000]; Default: 60) This property is only effective for cards based on Atheros chipset.
preamble-mode (both | long | short; Default: both) Short preamble mode is an option of 802.11b standard that reduces per-frame overhead. On AP: long - Do not use short preamble.
short - Announce short preamble capability. Do not accept connections from clients that do not have this capability.
both - Announce short preamble capability.

On station: long - do not use short preamble.
short - do not connect to AP if it does not support short preamble.
both - Use short preamble if AP supports it.