HomeKit WEMO Accessories - No Response

This sounds like a access point issue. If you are using a dual-band router and having difficulty locating your Wemo® devices, make sure that your mobile device is connected to the 2.4 GHz Wi-Fi router. Wemo devices may not show in the app or you might only see the Welcome screen if your mobile device is connected to the 5 GHz Wi-Fi. If you don't have a 2.4 WiFi router, you may need to get one just for setup purposes.

Hi TheJudgement,

Thank you for using the Apple Support Communities! We understand you're getting no response when attempting to access your HomeKit remotely. We'd like to get you the support you need to assist with this.

Your setup sounds like it will add a lot of convenience, and it also sounds like you've done some great steps on your own to try and resolve this. We just want to have you confirm a few more things, and then if the issue remains, we'd suggest reaching out to Apple Support next.

-Make sure Bluetooth is turned on, on your iOS device: Settings > Bluetooth.

-Make sure both Home and iCloud Keychain are turned on under Settings > [your name] > iCloud.

-Restart your Wi-Fi router and/or modem, if you haven't done that yet.

If the issue remains, use this link to get in touch with Apple Support next: Get Support

Best regards.

See my speculation about the using the Homepod Mini as a Home Hub here:

https://discussions.apple.com/thread/252214204

I have read that the Homepod Mini has a more advanced wifi chip than the Homepod so hopefully it won't suffer the same 5gHz problems as some have experienced with the Homepod.

Re-reading the OP's post this could be related to an issue that I experienced while on vacation. I found that if my iPhone was connected to the local wifi I couldn't control some of my Homekit devices at home. I had to disable wifi on the iPhone in order for it to work. However this seemed to be related to my installing a spare Apple TV at the holiday accomodation and linking to it with my iPhone via the local wifi. The ATV may have behaved like a Home hub and confused the Home app on the iPhone.

I realise that the OP experienced the problem with cellular connection as well as over wifi but my experience might point to some possible troubleshooting.