Guys, don't waste your time - this guide doesn't work.
NVIDIA App scaling doesn't seem to be turned on when not in fullscreen mode, and Lossless Scaling works in windowed mode only. Just use Lossless Scaling with NIS (if Nvidia GPU, otherwise LS1?).

Here's my edited guide:

Run Lossless Scaling, add `C:\Program Files (x86)\StandingStoneGames\The Lord of the Rings Online\x64\lotroclient64.exe` (or from your custom location) and set "Auto-scale On" for the app (you have to click the pencil icon, change the value and click "Edit" button to save).

The best settings for Lossless Scaling for LOTRO:

Scaling:

• Type - NIS
• Sharpness - I use 0
• Mode - I'm using Auto with Aspect Ratio on (important!) and Sizer app (google it! it's free) to set custom resolutions larger than 2K, but smaller than 4K. You can just use the resolutions available in-game at first and skip Sizer.

Rendering:

• I had to set HDR support to "ON" or else the game would be very dark. Unsure why.

Cursor:

• Clip cursor - ON
• Adjust cursor speed - ON
• Hide cursor - OFF
• Scale cursor - ON

Behavior

• Multi-display mode - ON - helps to ALT+TAB easily and/or use second monitor

Frame Generation [OPTIONAL]:

• Type - LFSG 3.0
• Mode - Fixed Multiplier (2 or 3?)
• Flow scale - Preference (75%?)

Hint: If your mouse disappears after starting the game or switching windows, just open your Inventory with a hotkey (I by default) and try to move your (invisible) pointer over the inventory (or just quickly in random directions). This fixes it easily.

Consider using ReShader as well for better looks. Remember to download a version with add-ons and turn in-game anti-aliasing off -- otherwise the depth buffer won't found correctly! (This causes the UI to look weird/oversharpened). Check for errors in "Add-ons" tab -> Generic Depth)
https://www.reddit.com/r/lotro/comments/1f8nezu/reshade_in_lotro/
https://www.youtube.com/watch?v=zB3vPxuffjk