My Lumia 620 disconnect wifi few times a day, few hours everytime, whenever I go to the wifi settings, either there are just wifi(without mine) with no signal or nothing at all. Restarting phone and router won't work
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Does the issue only happen with a particular Wi-Fi? Is the behavior same when connecting to other Wi-Fi networks? Try rebooting the phone. Don't forget to remove and replace the battery. If you have a phone cover, remove it for a while and connect the phone to the Wi-Fi to test it. Covers may block the Wi-Fi antena from receiving signals.
You may also delete the WiFi profile saved in the phone. Go to Settings, and then Wi-Fi. Tap and hold the Wi-Fi network that's having issues, and then delete. After that, reconnect the phone to the same Wi-Fi network.
It's also recommended to keep the phone software up-to-date. Check it by going to Settings, and then Phone update.
Failing still, try to restore the original settings of the phone. Go to Settings, and then About. Hit Reset device. However, this step will erase all data stored in the phone. Kindly create a backup under Settings, and then Backup. You may also transfer your media files to the PC via USB-MTP to avoid data loss. Consider resetting as your last resort.
Hope this information helps.