Downloading complete text message conversations over several years

I am needing to download the complete conversation from start to now between 2018 and 2021 with a specific phone number. I have never deleted this conversation thread in the past 4 years. i regularly back up my phone to the icloud. I backed up my iPhone manually yesterday to my computer to try to retrieve the messages dating back to 2018 to export them. Unfortunately the backup stopped mid June of 2020. I cannot find messages from before that. I need to retrieve the messages missing from dates between 2018-2020. Conversations with other phone numbers showed dating back to 2015. So it is not as if the backup stopped consistently for all conversations on that June Date. Just the conversation thread I need.

I have read online that the messages should be stored somewhere. I just don't know where to look or how to access them. Can someone please help?

Thank you!