Some devices may have problems with IPv6. Try disabling IPv6 in the router settings and see if that helps (Modifying the DNS server directly on the router could also be something that works). On the affected devices, go to the Wi-Fi settings, "forget" the network, and then enable MAC Spoofing if available, and reconnect by entering the password again. Try temporarily switching to a less restrictive security setting (like WPA) to see if that resolves the issue. Try connecting one of the problematic devices to a mobile hotspot (like your phone's) to check if the issue is specific to your network. Make sure DHCP is enabled on the router. This allows the router to automatically assign IP addresses to connected devices.