Draytek Vigor2120n-plus User Manual page 333

Open Wireless LAN (2.4GHz)>>Band Steering to get the following web page:
Available settings are explained as follows:
Item
Enable Band Steering
After finishing this web page configuration, please click OK to save the settings. Below shows
how Band Steering works.
Description
If it is enabled, Vigor router will detect if the wireless client
is capable of dual-band or not within the time limit.
Check Time.... – If the wireless station does not have the
capability of 5GHz network connection, the system shall
wait and check for several seconds (15 seconds, in default)
to make the 2.4GHz network connection. Specify the time
limit for Vigor router to detect the wireless client.
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Vigor2120 Series User's Guide