I have a BOINC dedicated Alcatel U3 / TCL 4034D with Android 6.0 that runs BOINC 7.18.1, but it loses network connection if it is not actively used. I've configured WiFi to always be on using Keep Wi-Fi on during sleep = Always. My current solution is to shut it off daily and restart it 5 minutes later automatically using Scheduled power on & off. It then usually keeps network for a few hours. It would be better if I could reboot automatically, e.g. each 6 hours. I there a way to do this? The current system with daily reboots works rather well since it can exchange data for a few hours every day.

I have another older Samsung GT-S6790N with Android 4.1.2 next to it, but that never loses connection.

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Unless jailbroken, you can't force a reboot without human interaction, unless you remove the battery, and have the ac adapter plugged in a timer.. Just set the wus to 0.5 days and an additional 0.6 days in settings, so your phone will buffer up WUs.