I've work a lot for understand this problem and found a solution.
Sms are sent through LTE when you are on 4g network.
You need to check the correct Apn for your operator and set it into section interface of the Gui.
Not all Apn allow to send Sms.
It is not enough that you are able to connect to internet.
For Tim correct apn is 'ibox.tim.it' and not the automatic set 'wap.tim.it'
Before set it, the problem was on this parameter that you can check only by CLI
>uci show simcard
Check parameter force_apn.
It must to be set as '-1'
It is not present into the gui but it very important and also sometimes this setting will be lost when you update firmware.
Set this by this command
>uci set simcard.@sim.force_apn='-1'
Check the apn and set it correctly into the interface section and everything will works good.
You can offer to me a beer