welcome!
0. is n810 nice?
1. what error messages do you get?
p7zip should open what 7-zip does, and i never had a problem opening tar with it.
a tar.gz file is exactly a tar that has been gzipped, so it's something only more complex than a simple tar.
it should make no sense the software to don't know how to handle a .tar while it does for a .tar.gz
more: tar is not compressed at all, while gzip has no "collecting" feature.
tar only collects, and gzip only compresses a single file.
i would like to support zip, but i didn't find a simple way to get it yet (in my spare time).
2. please try gzipping the .tar, so you get a .tar.gz and see if it works