You may want to make sure that your Wi-Fi settings match the recommended settings for OS X and iOS devices:
You can also use this article to troubleshoot your Wi-Fi connectivity:
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Thanks for suggestions, but recommended settings did'nt solve my problem.
Rather, I found the solution changing traffic priority parameters in router Quality of Service: this stopped WiFi disconnection (the reason is probably the presence a LAN connection to TV).
Below the procedure followed:
log into the D-Link DSL-2750B gateway (browser: http://192.168.1.1) --> Wireless Clients --> Advaced --> Advanced Wireless --> Qos --> WMM --> disabled --> apply