To place an object at a specific location with 8th Wall, you should use Lightship VPS: https://www.8thwall.com/docs/web/#lightship-vps.
This process involves nominating the location as a wayspot (if not already), making several 3D scans of the location, and placing the object relative to the 3D scan of the location in 3D modeling software or the A-Frame inspector. You can find full details on developing Lightship VPS experiences here: https://www.8thwall.com/8thwall/vps-bespoke. And a video tutorial here: https://www.youtube.com/watch?v=AY47_A6MsqU.
Alternatively, you might consider geofencing content within a close proximity of a specific coordinate. You can either use standard JavaScript location APIs, or the Niantic Lightship Maps module. With the primitive, you can assign a point at a specified latitude/longitude to the map, effectively guiding users to designated locations where they can activate the AR experience.
Placing content with coordinates alone is not supported out-of-the-box, though one possible solution could be to request the users location, calculate the distance from the latitude/longitude, and use that to scale the model. You could then use compass data to help orient the content.