@rwillats A dedicated wifi that you control is the only way to go. I like UNIFI products - they are designed to work from the ceiling and can be attached to a clamp to hang on a lighting bar or truss easily.
Get the biggest bandwidth you can - connect all computers via LAN and use the config software to scan for the best free frequencies.
The Unifi U7 has pro/max and long range versions if you want long range.
You will want a controller and router with that too - the cloud gateway ultra is the smallest and cheapest.
Together these give the flexibility, control and throughput you need to have reliable wireless on stage.
BUT I also doubt you will be experiencing issues with a sub net unless there is some really big config issues or the devices are connecting to different networks (many theaters have a public wifi, an admin and a tech - on all the devices make sure you forget all of them except for the tech wifi you are using. Next turn off the SIM based internet. iPhones have a feature that will switch the network to the sim card if it thinks the internet there is faster - this will mess up your wifi network too. Depending on the config of the in house system you can make manual IP addresses on your devices and have them reserved in the in house wifi matching the mac addresses (be sure to turn off the wandering mac address feature on ios/mac os.
Getting any settings changed on the in house network infrastructure is often impossible - ergot it is way easier to bring your own isolated network you can tune to your needs.
Fred