Airtel — Airtel 4g hotspot erratic connectivity

My parents are using Airtel 4g hotspot in Coimbatore, Tamilnadu 641010 since December 2018. During the last few weeks they are having trouble connecting to the internet. They visited Airtel office, DB Road with the device and sim. The resolution given was to change the Airtel sim. We have purchased a new sim. It seemed to work when used in the hotspot. My parents may have used 200MB of data. But connectivity is erratic. Sometimes it shows amber light, and other times green light. Now, even when the device shows green light, they are unable to access the internet using the laptop or mobile phone connected to the hotspot. They have recharged the sim card for 56 days with 449 Rs, and have not been able to use the internet for 2 weeks now. Since they are using airtel only in the hotspot they are unable to contact 121. I have tried to submit a complaint from another airtel number for network connection issues with Service Request No.C4GSP[protected]. We do not know where the problem lies and also have no access to register a complaint to the Airtel customer care.