One specific android phone is unable to receive texts from my iPhone

One specific android phone is unable to receive texts from my iPhone. I have an iPhone 8 with the latest software update. I can receive texts from this phone, but cannot send them to it. I immediately get a red "Not Delivered" message next to my text. I also get a red exclamation mark in a red circle next to the red "Not Delivered" message. My messages to this android phone are in green bubbles (not blue). The problem seemed to start when I switched to O2 as the carrier. The android phone in question is using "Giff Gaff" which also uses O2 as the carrier. I have spent a lot of time on the phone with both O2 and Apple and have yet to resolve the issue. Both parties indicate that the problem is not to do with them. I am able to send texts to other android phones. I am able to phone the android phone in question. I have tried many things including resetting Network Settings and resetting all Settings on my iPhone. I have also tried deleting all conversations with this android user and even deleted that person from my iPhone contacts list and tried manually typing the number in to the message to be sent. Still no joy. The android user has tried blocking and then unblocking my number. The problem also occurs on my Apple Watch which is the version with its own eSIM that can make calls and send texts independently (without my iPhone). Anyone know of the solution to this?