LB1121 - AT&T ?

AT&T SIM will work in the device, but it very much depends on which plan you're getting.  You pretty much need to get a data only plan which are more expensive than say an unlimited phone plan.  I started out trying to get their wireless internet plan which comes with their own 4G modem/router.  That worked but you cannot put that device in bridge mode so that you can use your own router.  The Netgear modems do allow you to bridge, but apparently that doesn't work with most carriers (including AT&T) unless they can give you a static IP address, which they can't/won't.

I wouldn't use AT&T BTW.  I've been dealing with them for a month now on this issue.  Their support is the worst I've ever dealt with.  Cricket is owned by AT&T and use the same exact network, but is a way smaller company and the support people are pretty good.  I just signed up for 40GB for $50/mo. with no speed caps.  If you exhaust your data allowance for the month you can purchase another 15GB for $10.  They have higher and lower plans as well (100GB and 20GB I believe).

I'm just running into the carrier grade NAT issue now I believe (see https://community.netgear.com/t5/Mobile-Routers-Hotspots-Modems/Re-opening-LB1120-Bridge-mode-issue/m-p/1909593).  So for my setup I'm still not operational.  But I have put both AT&T and Cricket SIM cards into the LB1121 and in router mode is seems to work great.