I only know this because I have one that's broken. I thought it was a firmware issue, but unfortunately, no matter what version I try, it still doesn't work. It's as if the 5G/LTE radio module was faulty. The only thing that works in my device is the GPS module, which shows the location nicely, but what good is that if a basic feature doesn't work 
Unfortunately, no manufacturer will tell you how to enable EDL mode. Everyone wants to throw a device away if it breaks. Despite this, I still believe that we should try to fix it, not throw it away!
Here are the photos. The voltage between the pins is 1.8V. Before powering the Zyxel, short these two pins for 20-40 second and only then apply power. Then the modem will present itself in OpenWRT as:
root@OpenWrt:~# cat /sys/kernel/debug/usb/devices
T: Bus=01 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=480 MxCh= 0
D: Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS=64 #Cfgs= 1
P: Vendor=05c6 ProdID=9008 Rev= 0.00
S: Manufacturer=Qualcomm CDMA Technologies MSM
S: Product=QUSB_BULK_CID:0410_SN:XXXXXXXXXXXX
C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr= 2mA
I:* If#= 0 Alt= 0 #EPs= 2 Cls=ff(vend.) Sub=ff Prot=11 Driver=(none)
E: Ad=81(I) Atr=02(Bulk) MxPS= 512 Ivl=0ms
E: Ad=01(O) Atr=02(Bulk) MxPS= 512 Ivl=0ms


I sincerely hope that if you succeed, you will describe here what you have done for posterity, am I right?