What are the parameters to determine whether this
modem is supported by my cellular carrier technology ?
My smartphone's sim card is 25mm long, 15mm wide
can i insert this sim card into a Netgear LB1120 modem ?
How complicated is it to configure this modem without
my cellular carrier support ?
Can i add an openwrt router between this modem
and my laptop ?
I'd appreciate your comments
I can answer questions #2 and #4:
#2. The LB1120 accepts a Micro SIM, which is 15mm x 12mm. If your smartphone SIM is 25mm x 15mm then that's a Standard SIM. You will need to either cut it down to the size of a Micro SIM and then use an adapter SIM tray for your smartphone, or go to your carrier and get a proper Micro SIM (and you will still need an adapter for your smartphone).
#4. Yes. The LB1120 has a single ethernet port which you can use as the WAN port for your OpenWRT router. You can configure the LB1120 to run in "Bridge Mode" to avoid double NAT issues. You could also possibly put your OpenWRT router into Bridge Mode to use the LB1120 as the LAN, but your OpenWRT router is going to be a more sophisticated router, so I don't recommend the latter configuration.