I have reported this same problem to Meraki support a few weeks ago. No progress and I am not expecting one. It is not easy for them to troubleshooting, but it makes no sense that this many users are reporting it. I am seeing it on all our Meraki networks in deferent GEO location, on different WAN links, using RADIUS/802.1X authentication. RADIUS servers are in the Data Centre. Problem is very intermittent, so not easy to capture. I could see  that Meraki AP is not getting RADIUS response when the Wi-Fi starts dropping. I also see affected SSID showing as low signal from the laptop while being away from it 3 meters while all other broadcasted SSID are full signal. Not sure if I need to indicate that we are seeing this on different Meraki models and firmware, updates didn't fix a thing. ... View more

We have also used GRP and plastic boxes for indoor APs to be used outdoors and not seen any problems with signal strength.  tested with the door open and closed without a difference in signal strength.    if your curtain is large and heavy,  there may be a metal mesh inside the layers for strength that you may have to consider.  if it is there, depending on the mesh size, it may or may not have an effect.  You know, just to muddy the waters ... View more