I earlier encountered the same problem when I updated to iOS 15 beta. The solution I found was to downgrade to iOS 14.x, and of course, the alternative way is much easier... retrieve your older iPhone from the garage and use it as a hotspot sharing router.
My iPhone 5s may not seem representable but it's still working and is able to connect my other devices including AirportExpress 1st generation, iPhone 8+, iPhone X, MacbookPro, Apple TV 3rd, iPhone 12 to the internet all at the same time.
By the way, I'd never replaced the iPhone 5s battery so I sometimes place it on an ice pillow.
g_nyquil
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