Caveat: you may need to set the Device Type of your source to “PC” for this to work.
- To use your smartphone as a remote, install the SmartThings app and add the TV into it.
- Press “🔇 1️⃣ 1️⃣ 9️⃣ 🆗”. The service menu should appear.
- Press 1️⃣ 2️⃣ 3️⃣ 4️⃣ to make the advanced menus appear.
- Go to “Option” → “Production Option“.
- Turn off “PC Mode Dimming Support“.
- To exit the Service Menu, turn the TV off and back on.
Thanks to u/will_work_for_vinyl for their slightly difference instructions which inspired mine.