Please submit a ticket here and allow support team to check for the connection logs.
https://ticket.peplink.com/ticket/new/public
Please make sure you attach a diagnostic report in the ticket and provide the MAC address for the device that having issue to connect.
Steps to obtain a diagnostic report from the device can be found using the following URL:
Things you can check as well to isolate the issue :
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Make sure country setting is correctly configured based on your location.
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Use a static channel 1 and verify whether it will help those device to connect: