Wi-Fi keeps disconnecting: dropouts and fixes

What the message looks like

Das WLAN trennt sich immer wieder, besonders in weiter entfernten Räumen oder zu bestimmten Zeiten.

The signal is weak at the edge of range and fluctuates, so the device loses the connection.
Interference on the channel in use, especially on 2.4 GHz, makes the connection drop now and then.
The power-saving mode of the Wi-Fi adapter briefly switches the connection off on a laptop.
The router has been running a long time without restart, is overheated or has outdated firmware.
The device switches badly between 2.4 and 5 GHz or between router and repeater.
Outdated or faulty Wi-Fi driver in the device.

Check when the router stops answering (Windows; on macOS/Linux use -c 60 instead of -n 60)
ping -n 60 192.168.178.1
Show signal strength, band and channel of the device
netsh wlan show interfaces
Create a report of Wi-Fi events (run as administrator)
netsh wlan show wlanreport
Show current IP settings and gateway
ipconfig /all
Check whether only Wi-Fi is affected: internet reachability
ping -n 20 8.8.8.8

Note when and where it happens: only in one room, only one device, or all devices. This narrows the cause a lot.
If all devices are affected, restart the router and check for firmware updates. Menu names differ by router.
If only one device is affected, update the Wi-Fi driver and switch off power saving for the adapter in its device properties.
Move closer to the router or improve its position. Walls and distance are the most common cause.
With a lot of interference on 2.4 GHz, pick a fixed channel (1, 6 or 11) or use the 5 GHz band near the router.
If drops are linked to a repeater, check where it sits. It should still get good reception from the router.

No. Occasional glitches can happen, but regular drops almost always have a cause such as interference, signal or driver.

That is only a workaround. If a restart always helps, check the firmware and whether the router has good ventilation.