I found the two screws behind the SN sticker, but having a tough time getting bottom plate off. Is there a trick to opening it? I have a new WWAN modem I’d like to swap out with. Thanks.
Figured it out without damaging the housing. Trick was to get the clips at each of the four corners after taking out the two screws underneath the sticker. Then it was fairly easy to pop out. 
Swapped out the Quectel EC25-AF (FA) for a Quectel EP06-A and more than doubled my speeds on T-Mobile (US). CA (Carrier Aggregation) works great too. Should have ordered it with the EP06-A in the first place.
Instructions below, hope they help 
p.s. you don’t need to remove the PCBA screw
Oops just realised you’d resolved. Left for archive purposes and future reference
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This is extremely helpful! I will keep this page bookmarked in the future just in case. Thanks!
I popped off the bottom-cover the wrong way, after looking at your instructions. It was relatively easy for me to get the side clips dislodged but then quickly realized there were clips at each corner. Once I pop two corners on one of the two “short” sides, it came off relatively easy minus the “stickiness” of the heatsink compound and sticker.