Deploying Excel web AddIn on development server

I created a Excel web Add-In using Visual studio 2017. It works fine from my desktop Excel when I deploy on my local host.

I published this Excel web Add-In and deployed on our test server IIS and saved the manifest file to a network share location.

Then tried to access this web Add-in from my desktop Excel, Add-In comes up with a distorted HTML, how can I fix this. Have inserted the images of correct form of Web Add-In and the distorted one I am seeing as below

Can anyone please help me on this as to how we can deploy a Excel web Add-In on a development server. and make it to work on my desktop Excel.

Thanks in Advance

Distorted html of Add-In as seen when I deploy to our test server

Correct html of Add-In as it should be