iPhone Loses Cellular Data when connected to CarPlay FIX

I have been battling this issue for a long time. Background: iPhone 12 Pro with iOS 14.x and an aftermarket Android headunit (without a SIM card) on a '14 BMW. The unit connect wirelessly to the iPhone for CarPlay function via Zlink. I have NEVER been able to get the wired connectivity to work. Worse yet, when I would connect to CarPlay, I would not be able to connect to the internet either from my phone or Carplay because the headunit needs the iPhone to connect to it via wifi to enable Carplay (plus the Bluetooth, too). Once connected, if you disconnect the wifi, you lose the carplay. The sometimes fix was to keep refreshing Safari or something until the phone asked if you wanted to ignore wifi and use cellular instead.

Fix: After reading many, many posts and complaints and compiling things, I figured out a solution to get Carplay functionality and maintain cellular internet.

1. Connect iPhone to Carplay via bluetooth and wifi. Make sure Carplay is up and working on Zlink.
2. Go to wifi and select the Android headunit wifi "hotspot" you are connected to. Hit the (i) and scroll down to see your Configure IP address. See first pic 1. Write down the top IP address and the Subnet address.
3. Change Configure IP to Manual by clicking on the word "Automatic >"
4. Change the IP Address from 0.0.0.0 to whatever your IP address is (prob starts with 192.x.x.x)
5. TYPE the Subnet Mask to whatever it was from Automatic (usually starts with 255). Doing this step will turn the word "SAVE" at the top of the screen to blue letters.
6. Do NOT put anything under Router.
7. Hit SAVE. When you go back, you will see little yellow words under the Wifi name of your Android unit that says "No Internet" If you see this, SUCCESS. your carplay might still be connected or you might have to reconnect. But at this point, your iPhone know to NOT TRY TO GET its internet signal from your headunit and will rely on Cellular.

This worked for me and I hope it works for you, too.