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Where is the location of the modem/router? Did you notice the changes of the light status of the modem/router, when the connection for your 2.4GHz network isn't working?
Aside from turning off and on the device, have you tried to factory reset it? If not, please try to factory reset the device. Performing this, the settings will get back to default and you may personalise it again.
If the issue persists even after the factory reset, please let us know and will check this further.