Try applying a bit of pressure with your fingers on the laptop base above where the SSD plugs into the systemboard and see if the SSD is detected on startup.
Bit hard to explain exactly where but here is an image of the SSD location, taken from the service manual, (p.58), so hopefully you can work it out.
(click on image to enlarge for better viewing)
If the SSD is still not being detected by trying this, you may have to try inserting another SSD to determine whether the problem is in the SSD or in the systemboard
If it is being detected when you do this then perhaps the motherboard SSD connector contacts or the SSD board contacts may be dirty etc. I realize that you have removed and replaced the SSD and usually this will clean the contacts but if it does detect the SSD by doing this then obviously there is something wrong that need to be further investigated
With a strong light and a magnifying glass, visually inspect the SSD connector on the systemboard (and also check the SSD connector contacts) to ensure that it is OK and does not appear corroded etc
If the systemboard connector contacts appear corroded etc, and you want to clean them (Isopropyl Alcohol 90%+) ensure that you disconnect the battery from the systemboard before doing any repair work.