I have full bars where I live and 4G LTE and the SIM used to test the phones was my own, which is working perfectly on my Zuk Z2 plus.
At first, I thought it was a GSM lock. Upon researching online it seems that the LG G5 gives you a straight message warning you when the phone is locked and askes for a GSM unlock code, that isn't happening here.
I also check all frequency bands available where I live and the phone indeed supports them all.
A third possibility that was considered is that the IMEI was blocked by a local government agency because they want to prevent people from importing phones directly, however, they have a tool where you can check if the IMEI is blocked, so I did and it is not blocked.
I'm still trying to figure it out. Maybe there's some sort of software shenanigans going on.
Next, I'm gonna try to jailbreak the phones and install a different operating system. Has anybody done that to this phone?