www.videolan.org
sudo apt install libdvd-pkg
sudo dpkg-reconfigure libdvd-pkg
As far as I can see, running the two commands above in Terminal should solve the problem
The above installation and configuration will enable the dvd player to play commercial dvds which have been released with the Content Scramble System (CSS), however, it won't help if the dvd player cannot detect the dvd in the first place.
Working dvd players will normally detect the dvd. Then, if they don't have the decoding provided by the libdvd-pkg software, they run it with a blurred or severely distorted screen making viewing of the dvd's content impossible for normal viewing. The case for the OP here is the failure to detect the dvd in the first place as I understand it.
This "failure to detect the dvd" problem is very familiar to me. It may be a software problem, which could be misbehaving "firmware/software" in the dvd player itself, or an actual hardware problem such as the failure to align the laser onto the dvd disk, but since CDs are able to be played, I suspect the "software/firmware".
The way in which I've dealt with it is to replace the dvd player because I wanted a reliable one. Over the last 25 years, I've been through at least one dvd player a year over numerous computers and have tried all sorts of brands. There are lots of no-name brands available, but I learnt to only buy named brands, and not buy the cheapest of them.
Edit: I have assumed in the above that the basic dvd software is installed, i.e. libdvdnav and libdvdread. If they aren't installed, it's possible they have been implicated in dvds not being seen.