I worked with images from a Xiaomi Mi Sphere camera, and they were 'dual fisheyes' - two fisheye images, each with slightly larger than 180 degrees field of view, next to each other. I strongly suspect that your images are similar to that. Such images can be converted using panotools-based software like hugin - you need to go through a few steps to get the result:

• create a PTO with twice the same image from the camera, projection fisheye
• the images need to be horizontally shifted in opposite directions
• and masked with a circular lens crop, one fitting the left and one the right image

Then you search for control points, optimize and stitch to full equirect output. Once you have a working PTO, you should be able to reuse it for other images from the same camera, so you only have to go through the creation of the PTO file once - like a calibration. Care to share a test image? Then I can give it a shot. You may be interested in this thread on hugin-ptx which has a concrete PTO for the Mi Sphere.