Hello,
I could you contact your ISP support to enquire if they see connection attempts on their side and if they have the modem IMEI registered in their database?
If they report no issues, the next thing to try would be to upgrade modem firmware. You can find instructions here. <ID> in your case would be 1-1.4 and <VERSION> - SLM750_VE_4_0_15
Best regards,