makeWebRequest works on simulator but not on device with -300 responsecode for https

I'm trying to get some API data from URL https://apis.data.go.kr/B552584/MsrstnInfoInqireSvc/getNearbyMsrstnList

On simulator everything is fine (with responseType HTTP_RESPONSE_CONTENT_TYPE_TEXT_PLAIN, I can get 'SERVICE ERROR' xml text successfully)

But when I sideload and run this widget on my Epix it doesn't work with responsecode -300 (NETWORK_REQUEST_TIMED_OUT)

It doesn't look like actual timeout since the responsecode comes out almost instantly(less than a second),

and with OpenWeatherMap API(https://api.openweathermap.org/data/2.5/air_pollution) everything worked fine.

I looked for similar cases as mine and found out there may be HTTP/HTTPS issue, or HTTPS with self-signed cetificate issue.

But I think those issues are not applicable to my case. (I'm using HTTPS with not self-signed certificate)

How can I get through?

I also attach source code below, but I think there's nothing to do with code itself(too simple to work on it)