You have not mentioned the following, which is essential for arriving at a solution
- What happens when you connect your iPhone to your Computer?
- Do your hear any sound?
- Are you able to see your iPhone as a device under System Preferences > Network > Network Name?
- Do you have a strong internet connection on your iPhones 6 & 8?
- Are you able to connect your Mac to internet using WiFi signal by enabling Hotspot on iPhone 6 & 8?
- Are you able to connect your Mac to internet using Bluetooth signal by enabling Hotspot on iPhone 6 & 8?
Suggestions: Use Certified USB Cable. Uncertified ones may work on some iPhone for limited time and may not work at all on other iPhone models.
Tested cable does not mean it is genuine.
Identify counterfeit or uncertified Lightning connector accessories ...
https://support.apple.com/en-us/HT204566
Identify counterfeit or uncertified Lightning connector accessories
Apple recommends using only accessories that Apple has certified and that come with the "Made for Apple" badge. Learn about the issues that counterfeit or uncertified Lightning accessories can cause and how to identify these accessories.
If you use a counterfeit or uncertified Lightning accessory, you could see these issues:
- Your iOS device could become damaged
- The cable might be easily damaged
- The connector end might fall off, get very hot, or might not fit properly into your device
- You might not be able to sync or charge your device
Only Apple makes the USB-C to Lightning Cable. There are no "Made for Apple" USB-C to Lightning cables.
Compare packaging and writing on the cable
To identify counterfeit or uncertified cables and accessories, look carefully at the accessory's packaging and at the accessory itself. Certified third-party accessories have the MFi badge on their packaging:
Some accessories might have a previous version of the MFi badge:
An Apple Lightning to USB cable has "Designed by Apple in California" and either "Assembled in China," "Assembled in Vietnam," or "Indústria Brasileira" on the cable about seven inches from the USB connector. You'll see a 12-digit serial number at the end of this text.
A certified, third-party Lightning to USB cable usually has company branding on the Lightning-connector end.