I am unable to use International Roaming and Airtel number is not working

I have moved to US during the end of the December. Before I start, I have activated my mobile number into an international pack and i'm paying the monthly bills as well from here. I have switched off my Airtel SIM in US and whenever is required I will on my Airtel SIM and the same was working fine. But, for the past two weeks, if I on my SIM it says SIM was not allowed. I have also checked the same with switched off my mobile phone and removed and reinserted the SIM. Still, i'm getting the same issue. I have dropped an email to Airtel and Customer care and had a Chat with the Airtel representative through Whatsapp. But still facing the same issues after i made the all changes suggested by the Airtel. I'm staying in Denver, Colorado and please advice how shall i proceed further.

Thanks.
Saravana Kumar. N