In my recent experience of Qatar at BKK I was told check-in opens 4 hours before departure. I've also read that a real paper boarding pass is expected by Qatar's boarding processes, so I'd recommend heading to a check-in desk at T minus 4 to get your two boarding passes. The desk should also be able to change your seat on the BKK - DOH leg if there are still some blocked seats showing; for some reason the Bangkok flights usually have all J seats blocked, but I know these can be allocated by check-in staff (I've done it myself) or perhaps over the phone.