App not working on home wifi

I downloaded an app and for months it has not worked properly. I always get the notification: "Server Error. There are issues connecting to the app. Please try again later."

What I found out is that it only doesn't work when I'm connected to wifi at home. But when I'm connected to public wifi, or someone else's wifi, the app works perfectly fine. I haven't tried with data yet since data is super slow.

So I believe the problem is not with my phone or the app, but with my wifi at home. Could it be the DNS? Or IPV6? Please help.

Notes:
(1) I own an iPhone 5S and everything is up to date
(2) I have messaged the app developers and my internet service provider but none of them have helped me
 
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