First of all you'd need to make sure that coturn is in the "certgroup" group. Or it won't help.
Secondly, those are just the two files. "certgroup" might still be unable to open them if the directory permissions don't allow opening the directory in the first place. And we can't tell from just the two lines. Make sure all the parent directories also allow traversing to that location. "rX" are required to read and open a directory.
However text files shouldn't have execute permissions. And you've set "rwx" for the group. That doesn't break anything, but "rw-" would be the 100% correct choice there. And other accounts shouldn't have read permissions on the private key. That should be "rw-rw----"
(I'm not sure why you have to do all of that stuff manually... Lot's of frameworks or operating systems come with letsencrypt/certbot and some sane default settings. If your way of doing things doesn't work out, you might want to read a tutorial for your operating system or consider a more standard approach.)