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Most people have a homeprod… Some of us also have a homeLab! (Modified from an old IT saying)
Fail over/dual internet access can be tricky. I’d first go over the configuration so that you are using the correct gateway and correct dns, etc.
Then, if your uplink is all sorted out, then most likely your cloud SIP provider is whitelisting your inbound IP address from your current ISP. you might need to give them the IP addresses you are using with your mobile provider. It’s also possible that your mobile provider is doing CGNAT or something odd that the cloud SIP provider can’t deal with gracefully.
If i were you, I’d try to get a tech on the phone with you to do some troubleshooting and isolate where the issue is.
Don’t tell anyone, but sometimes at work I’ll buy a shorter term service contract on hardware I’m planning on taking home when they roll out…