Can’t connect older iPhones to mobile hotspot?

I am currently trying to connect an iPhone 3GS (iOS 6.1.6) to my iPhone 11 (iOS 15.2) and it will not connect. It will say “Unable to join this network” forever. I have tried resetting network settings on the iPhone 3, it does not show up when trying to connect the phones with bluetooth. My iPod Touch 4 can connect to my 11, however. This seems to happen with all of my older iPhones, even newer ones like the 5C. Does anyone know how to fix this?