You cannot upgrade/update Apple mobile devices using a cellular phone data connection through a WiFi hotspot.
Apple device upgrades/updates need to be done over a direct WiFi connection from a physical router box that accesses the Internet from an Internet Service provider.
You will need to find a direct wireless WiFi MODEM/router box network connection at places that offer free WiFi or a family or friend's home the uses a direct Wireless, WiFi MODEM/router box network connection.
OR
Search online to locate and call contact your nearest, “local” Apple Store location to try/attempt to make an appointment to get your Apple iPad upgraded/updated to its latest iPadOS version by Apple
OR, alternatively, use the “Get Support” link, at the upper right hand corner of every Apple Support Community Webpage to attempt to schedule an appointment with your nearest Apple Store location.
Apple Store employees will be glad to help/assist you.
Best of Luck to You!