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Are you able to connect your other devices to the WiFi of the Huawei modem? If they are able to connect, then there's nothing wrong with the modem.
We need to focus on the configuration of your Unidem app cam 35. Since you were using the same modem before with a different ISP, means the configuration might be different. Have you tried resetting the Uniden cam back to its default settings? You might need to reconfigure the settings again.
If it's still not working, please provide us the screenshot of the error message for us to analyze this further.
Thank you for the information. When you tried to connect the appcam using the application, does it ask for the WiFi password? Or does it give you a straight error which says "failed to connect"?
Have you contacted the manufacturer of the Unidem appcam 35? What troubleshooting have you done with them?
Please provide us screenshot so we can analyze the situation further.
Proven the fact that other device(s) is connected through wireless connection there is a chance that your camera requires certain modem specification; but if your camera provider advised you to allow the port forwarding of Huawei HG659.
You can visit this link for more details. Port Forwarding on the HG 659
Let me know how it will go.