I have built a newer system recently so I had to reinstall lots of stuff.
I installed the LAME ACM codec tonight, which makes it available to Windows (at least W10 recognizes it).
Reaper can use it fine.
Tried the same render in RB.
RB shows the option as available but fails with a 'Cannot get Stream size' error.

Wish I could offer more.
Not sure why, even when available to the OS and other apps, RB fails at using it.
confused

image 1 - Codec available in W10
image 2 - Reaper using it
image 3 - Resulting successful MP3 file in Realband folder using Reaper
image 4 - LAME available in RB as a compressed format option
image 5 - End result error in RB when trying to use the LAME codec

Attached Files (Click to download or enlarge) (Only available when you are logged in)
WindowsCodecLameW10.jpg (59.95 KB, 109 downloads)
LameRenderReaper.jpg (100.58 KB, 110 downloads)
ReaperLAME_render.jpg (34.22 KB, 109 downloads)
LameW10.jpg (42 KB, 103 downloads)
RB_LAME_Error.jpg (31.11 KB, 105 downloads)
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.. I do not work here, but the benefits are still awesome