Devices vary greatly, but on some of mine, inside the Developers Options, there is an option to force he device to always maintain a data connection even if Wi-Fi is available.
Generally, a device will automatically stop most cellular data flow whenever it can connect to a Wi-Fi source.
This is to conserve data, as data is usually the expensive and limited portion of a cellular plan.
Perhaps this setting can be used by you for your purposes.
If you have not opened Developers Options on the device, you must first unlock them.
The method to do this varies by device {of course, right?} but is usually quite easy.
The general gist of it, at least from the device that I am presently using for this answer, is:
Settings
System
About phone
Build number
{tap build number 7 times}
A small toast message will appear to let you know that 'You Are Now A Developer!'
Now back out to the homescreen, and:
Settings
System
Deveeloper options
Turn the switch on, and scroll through to find
Mobile data always active
Hopefully this, along with the suggestions from
@Hadron , will achieve the desired results.