Fredde thanks for your input.
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That's the way it worked for me.
-Check the time of the router and adjust it if necessary (this was my problem)
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-Reserve for every client an IP from the Home tab
-Create a schedule: Management tab - Schedule - Add Rule
-Create a IPv4 Rule: Feature tab - Firewall - IPv4 Rules -Add Rule
For example Source 192.168.0.2-192.168.0.2, and destination 0.0.0.0-255.255.255.255, and port range 1-65535.
Set Source at LAN, Destination at WAN and Port Range at Any. And select the schedule that you created under Management tab -> Schedule.
Importend:Select in the drop down Rule list ALLOW or DENY for the created rule.