I found the following on the net regarding iOS 11 and greater disabling usb data after 1hr of use due to security concerns. I’m not confident this has anything to do with my issue as my connection drops are earlier than 1 hr.
“iPhones starting from iOS 11 will terminate the USB data connections after one hour by default to improve security. This can easily be changed via:
Settings > Touch ID/Face ID & Passcode > USB Accessories > ON”
https://openwrt.org/docs/guide-user/network/wan/smartphone.usb.tethering
Mine is now turned on as it was off for usb accessories.
.