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What I can suggest is to focus NOT on solving the issue, but finding more info/patterns about the issue. Take notes of everything you find out. Here are some examples.

- test mobile app with disabled WiFi (Disabled WiFi => you need to use Mobile Data => different network)
- test different devices at home
- test different homes (for example if user1 has the issue, then his/her boss can try to reproduce the issue at boss's home).
- when did the issue start -i am guessing since the very first try to work from home
- who else is affected - i am guessing nobody else

So if you think about it in term of points of failure - you have 3 such points - the devices you are using, the networks you are using, and the provider you are using. If you try a different location and are not able to reproduce this issue, it means that you are using different devices and a different network, but the same provider/ same app. I.e. not a provider issue.
These are all examples. In your case it seems likely it is not a provider issue, but still you need to rule-in or rule-out a device issue or a network issue.