First off. I'm sorry you have to deal with this. My company went the same route with before I came on to 'save money on costs' by going Wi-Fi only with our fleet. Frankly they are an absolute nightmare to manage. I have been slowly migrating our fleet to 4G-5G but that is of little help to you.
Honestly, it took a cost-benefit analysis written down on paper to management before they realized while there was cost savings upfront the back end support and updates quickly overrode that.
Sadly many of the answers I'm about to give are not going to be a great solution, just a stop gap measure. Here has what worked for me in the past, and also relies on how much your field ops have started to use the devices for personal usage and what type of devices they are.
- Get a pool of swapable devices and keep track of what is, or is not updated. You'll just need to juggle and re-deploy the devices to the field.
- Come up with a policy with management to make software updates/downloads part of the normal work flow process.
- Do the employees bring the devices home with them? Most people have wi-fi at home, and while this is not ideal this is how I get through most of my problems.
- If they are cell (aka cell phones) my carrier does not charge for os updates due to the fact they help secure the system. You might want to check with your account rep/sales (if you have one) if this is the case.
- Finally, if possible use this as a selling point to upgrade your fleet with management. It may sound cliche but if they are job critical devices it might be time to make the case that they are connected 24-7.
I know these are not the best answers, but it is something I've had to deal with at my location. I hope this helps you find a few solutions that will work for you.