You can try to set it in /src/index.js, You can bootstrap it, with the following code.
I think turning on keepAlive is necessary. Hope this help, for any detail setting, feel free to leave me message.
bootstrap({ strapi }) { strapi.server.httpServer.keepAliveTimeout = 35000; strapi.server.httpServer.headersTimeout = 36000; strapi.server.httpServer.keepAlive = true; },