How can I force iMessages to send instead of texts on airplane mode?

I’ve encountered this problem a few times when travelling. When I travel I always put my phone on airplane mode and use imessage or whatsapp for communication to avoid excessive charges for data roaming, sms, etc.

I’ve noticed that when I send an iMessage to a person, while briefly disconnected from wifi, it tries to send as a text instead. I‘m aware you can disable this feature, but I always forget until after this problem occurs. So now I’m stuck on texting only with this one specific conversation. I can iMessage other iphone users but this one convseation is stuck in limbo land.

An an option is to disable iMessage and then reactive it. However, I recall when I was travelling previously I attempted this only to find out that in order to active it you need to receive an activation code via SMS. But I’m in airplane mode.

My main question is, is there any way to FORCE my messages to be sent as iMessages to rectify the problem with this one convo? While apps like whatsapp are easy to use, iMessage is just more convenient and I’m sad I can’t use it right now.

Thanks for reading.