Grameenphone 4G Not Working On Vivo Y5

I am facing a continuous 4G network problem while using my Grameenphone SIM in my Vivo Y5 phone and I request your support to resolve it. Even though I am using a Vivo Y5 with a Grameenphone SIM, I am not getting proper 4G service and I cannot use mobile internet normally. The connection is unstable and the 4G service does not work as expected, which affects my daily use of the phone. I have tried to use mobile data many times but the problem continues and I could not solve it by myself. I kindly request that you check my line and the 4G service quality on Vivo Y5 with Grameenphone and inform me what the exact problem is and how it will be fixed so that I can use stable 4G internet on my phone.