First decide a directory on your Synology NAS where to store the Restic repository. I’ve chosen /volume1/restserver.
Download rest server for linux armv7l from the rest-server releases page and decompress it to the designated dir above. Something like:
wget -q https://github.com/restic/rest-server/releases/download/v0.10.0/rest-server_0.10.0_linux_armv7.tar.gz -O - | tar -C /volume1/restserver --strip-components=1 -xzf -