Unrar and unzip installation:

opkg install unrar unzip

UNRAR 5.50 freeware Copyright (c) 1993-2017 Alexander Roshal
Just simply change dir and use short paths.
First cd to your directory with parts of archive:

cd /home/Downloads
unrar x archive.part01.rar

If you want run unrar in background use nohup and & on the end.

nohup unrar x archive.part01.rar &

After all read nohup.out for possibly errors.

Do not use full paths:

unrar x /home/Downloads/archive.part01.rar

Another possibility is low memory, you should create SWAP partition.