For some reason, this update is not downloading on my iPads?

The Rapid Security Response (RSR) Update is delivered as an OTA update. The update is dowloaded and installed via WiFi. Unlike full firmware updates, an RSR cannot be installed using a Windows PC or Mac computer. 

About Rapid Security Responses for iOS, iPadOS, and macOS - Apple Support

If you have a beta profile installed, ensure this has been removed from your iPad - as, otherwise, installation of production software updates may be inhibited. These support page provides the necessary information to remove the beta from your iPad:

How to uninstall beta Software - Apple Support

You cannot use any smart phone WiFi hotspots to download.install Apple iOS/iPadOS upgrades/updates.

You need to be using a dedicated home Intenet network service provider and getting your Internet connection througn using a dedicated Wireless/Wifi MODEM/router box.

If you do not have a dedicated home Internet network service/service provider, you need to find a outside location that does use a dedicated and free Wifi Internet service, like coffee shops, restaurants, donut shops, book stores libraties, etc, to do yout own iOS/iPadOS upgrades/updates, using their free WiFi services by connecting to their own WiFi network MODEM/router

Altenatively, you can use the Support link, in the far upper right hand corner of this Website area, to make an appointment with your nearest, “local” Apple Store where employees will be able to upgrade/update that iPad for you.

Best of Luck to You!