Authentication failure trying to add Transmission DL client

Sonarr version (exact version): 3.0.8.1507
OS: Windows 10
Debug logs: https://pastebin.com/praw7nSt
Description of issue:

Hi there!

Just installed Sonarr on Windows 10, but can’t add a Transmission DL client running on a Keenetic Giga router. The Transmission per se is running fine, I can access it in Chrome from exactly the same Windows PC and from Transmission Remote app on Android using the same credentials. But when I try to add it to Sonarr, I get this:

I can see that it requests path /transmission/rpc but it seems to be a wrong path (GETting it from the browser results in 405 as well) and I can see no way to enter any other path when adding the DL client. What am I doing wrong?

PS Here’s an rpc-related excerpt from transmission’s settings.json: https://pastebin.com/7ckcKuSa

Did some digging and found out that another torrent monitoring software - TorrentMonitor has exact same problem with the Transmission client bundled with Keenetic routers firmware. Apparently the Transmission variant bundled with Keenetics is somehow stripped down and doesn’t have complete rpc support.