Q: text message forwarding code not stick after ALLOW

Hello Strgzr,

Thanks for using Apple Support Communities.

I see from your post that when you are setting up SMS forwarding on your iPhone, the toggle switch keeps switching off.  First I suggest signing out of Messages on both of your devices that you're trying to set up.  After signing out, please reset your network settings on the iPhone, and then sign back in to Messages and set up SMS forwarding.

1. Tap Settings > Messages.
2. Tap Send & Receive. 
3. Tap your Apple ID, then tap Sign Out.

Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support

Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

If the issue persists after following these steps, please let me know.

Take care.