Intermittent slowness in website

Question

We have a website that is accessed through the web and also through the android apps given to the sales team. It mostly works fine, and the response is also fast. Of late we noticed (could have been happening earlier also, may have gone unnoticed), but now maybe the frequency has increased. Sometimes the app keeps waiting and never moves forward showing the waiting cursor. Mostly this happens when the data to be uploaded is large. However, we have tried with 5G phones also, and still this doesnt resolve the problem. This is definitely not an android problem, its an API problem which is written in C# with SQL server backend. These requests do show up on the "Failed request trace" as waiting for more than 5 minutes. Server is located in Amsterdam whereas most users are in India, for cost reasons. I tried to see if any exception is causing this, but seems no exception is triggered in these cases. Could be a server load issue, because the number of failed requests goes up during times of load.

I have attached the failed request report but cant figure out from that exactly what keeps those threads waiting. Increasing the number of worker threads doesnt help. Server is automatically restarted every 4 days. Utilisation of the server remains low even during high load so that aspect is also ruled out. Database is not hit in these cases at all, so problem seems to be somewhere in the coding.