It looks like this function is only available in X1 series. I’ve tried SD printing from a 3mf file, but it says check default AMS mapping and nothing happens after pressed print.
The LAN only mode download is insanely slow, it’s like dial-up DSL speed! I still don’t get why print job needs to go through Bambu’s cloud in order to have basic printing functions. I can’t use the cloud due to confidential projects.
I did print with AMS from SD prior to using LAN only. Yes it says check AMS mapping, counts down (5s) and then starts to print. Export your file from the slicer as gcode onto SD, then there is no ‘check AMS mapping’, and there is no need to unpack the .3mf file. The printer goes straight to print.
I use LAN only since several month. Yes its slow to upload the files (100kB), but if you print over the cloud, you only see the fast upload to the cloud. Then the printer will download the file from the cloud and that is eqally slow as the upload in LAN only mode. The difference is: you can’t see it.
I have another printer with a Duet Wifi mainboard. It uses an ESP8286 for Wifi and goes with 250kB. Not fast, but 2.5x faster then the P1 which has ESP32.
My advice: If the gcode is only up to 30-40MB go with upload from slicer, if its bigger export as gcode and use SD transfer.
I’ve tried SD gcode printing too, it can only print from AMS A1 no matter what you pick in silicer. Were you able to print multi-color from SD gcode?
Some Reddit post says, the only way to print AMS multi-color from SD is using 3mf. It hasn’t worked for me yet, my P1S didn’t show a countdown, just print or cancel buttons.
I’m using LAN only, but upload gcode files with ftp transfer all the time and are using multicolor. Uploading with ftp or copying to SD should be the same. I start the print from the printer LCD and its using the AMS fine.
The commands to change the ‘tool’ aka Spool are in the gcode. Even the use of the first spool is a tool-change command in the gcode. And in the 3mf is the gcode.
The countdown will be a firmware thing. I’m on 1.2.x firmware still. The downsides reported from users outweighted the new features introduced with new firmware versions.
Sorry to bump this older post, but it seem directly relevant to the issue I’m facing - and I’m just seeking confirmation as to if my thinking is correct (and if it is, clearing up the issue for others as well).
I’ve run into an issue with the AMS2 on P1S whereby if there is only one filament defined on the prepare tab, even though it was configured at time of sending the file to the printer (send, as in to the sd card via LAN, not print via cloud) to use the ABS in slot 4 of the AMS it will use slot 1, which is PLA … 
I baby sat the printer and caught it in the act of misloading as I suspected it was loading the wrong slots after some queued PLA prints came out in the … wrong colours 
I then re-loaded the project, inserted filaments 1-3, and configured it to use filament 4 as the active one for the print (again, slot 4 ABS) and now the printer correctly used that slot when loaded from the sd card. It seems to me that when loading a print from the SD card that the P1/AMS2 will blindly/dumbly just use the filaments as sequenced in the prepare → project filaments section, regardless of the type. i.e. I think the “mapping” part that is mentioned here (although in relation to the X1) only works on the P1 if you printing directly via Bambu Studio or Bambu Handy app… which unfortunately makes my P1S way, way, way dumber than my A1 
I mean, seriously… the printer didn’t even baulk at using PLA when the print job asked for ABS… what is the point of having RFID reels? 
That part I really am surprised at.