unrar-nonfree can be installed from Debian armhf repository (it's simply called unrar there). For example, in Raspbian Buster you can run:

wget -qO - https://ftp-master.debian.org/keys/archive-key-10.asc | sudo apt-key add -
echo deb http://deb.debian.org/debian buster main contrib non-free | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get install unrar

Don't forget to disable the Debian repository once you're done: it's not recommended to upgrade your system while having a Debian repository as a source.

An alternative is to install 7-zip with RAR support (though it may not work on some archives):

sudo apt install p7zip-full

unrar-nonfree can be installed from Debian armhf repository (it's simply called unrar there). For example, in Raspbian Buster you can run:

wget -qO - https://ftp-master.debian.org/keys/archive-key-10.asc | sudo apt-key add -
echo deb http://deb.debian.org/debian buster main contrib non-free | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get install unrar

Don't forget to disable the Debian repository once you're done: it's not recommended to upgrade your system while having a Debian repository as a source.

An alternative is to install 7-zip with RAR support (though it may not work on newer or password-protected archives):

sudo apt install p7zip-full