Diagnosis
Your phone or device likely lost the connection because the router's 6 GHz band (Wi-Fi 6E) is either disabled or its settings were changed. The 6 GHz band has stricter regulatory controls and can be configured to turn off under certain conditions in the router's settings.
How to Fix It
Log into the router's admin page. Go to Advanced > Wireless > Wireless Settings. Look for the 6 GHz band section. Ensure the 'Enable Wireless Broadcasting' checkbox is ticked. Also, check the 'Control Channel'. If it's set to 'Auto', try manually selecting a channel like 37 or 93. The 'Enable SSID Broadcasting' box must also be checked for it to be visible.
If That Didn't Work
The 6 GHz band uses something called Automated Frequency Coordination (AFC). If the router thinks it's in a location (based on the country/region code you set during setup) where 6 GHz operation isn't allowed or needs special coordination, it may disable the band. Go to Advanced > System Tools > System Parameters and verify the Region is set correctly.
Heads Up
Not all devices support all 6 GHz channels. Some older Wi-Fi 6E clients only support the lower part of the band. If you manually set a control channel above 149, some phones might not see it. Sticking to channels 37-117 is generally safest for broad compatibility.