Jenkins not using my custom update center

Question

My Server is running behind a proxy. I have deployed jenkins a tried to install my required plugins but got connection error:

Failed to download from https://updates.jenkins.io/download/plugins/echarts-api/5.3.0-2/echarts-api.hpi 
(redirected to: https://ftp.belnet.be/mirror/jenkins/plugins/echarts-api/5.3.0-2/echarts-api.hpi)

I have checked and found that I my proxy allow me connecting to another mirror (ftp.halifax.rwth-aachen.de)

I have downloaded the default config uplaod-center.json, edit it by replacing updates.jenkins.io with ftp.halifax.rwth-aachen.de and configured my file in the Update Site screen.

I have restared jenkins but I'm still getting the same connection issue to updates.jenkins.io. Am I missing any thing here?