I use Textra also and your video texting issue probably isn't caused by the Textra app. It is more likely the carrier limitations for texting. Unfortunately, videos are often limited by the carriers to 1MB-3.5MB. If your video(s) is larger than that, you would most likely not be able to send it irrespective of which messaging app you were using.
You can send much bigger (20MB) videos using Google Photos. Info for sending video through Google Photos can be found
HERE.
You can also email larger videos, but they are limited to a similar size to Google Photos. I think Gmail and Yahoo limit files to 25MB and Outlook/Hotmail to 20 MB.
If your video is larger than the limits set by the carriers or the email provider, you will have to reduce its size. Info on doing so may be found
HERE.
There have also been issues with Android users being able to sent videos to Apple users. Suggestions for sending video to an iPhone user
HERE.
Good luck.