Here's an nslookup with wireguard disabled:
LibreELEC:~ # nslookup p16-ad-site-sign-sg.tiktokcdn.com Server: 8.8.8.8 Address 1: 8.8.8.8 dns.google

And here with 1.5.3 beta2 built in mullvad enabled:
LibreELEC:~ # nslookup p16-ad-site-sign-sg.tiktokcdn.com Server: 100.64.0.63 Address 1: 100.64.0.63 nslookup: can't resolve 'p16-ad-site-sign-sg.tiktokcdn.com'

LibreELEC:~ # nslookup youtube.com Server: 100.64.0.63 Address 1: 100.64.0.63 Name: youtube.com Address 1: 142.251.174.91 qc-in-f91.1e100.net Address 2: 142.251.174.93 qc-in-f93.1e100.net Address 3: 142.251.174.190 qc-in-f190.1e100.net Address 4: 142.251.174.136 qc-in-f136.1e100.net



Interestingly enough even specifiying google dns didn't work:
LibreELEC:~ # nslookup p16-ad-site-sign-sg.tiktokcdn.com 8.8.8.8
Server:    8.8.8.8
Address 1: 8.8.8.8

nslookup: can't resolve 'p16-ad-site-sign-sg.tiktokcdn.com'





I tried manually adding a config file for mullvad in the format you suggested and I was able to nslookup the tiktok cdn just fine:
LibreELEC:~ # nslookup p16-ad-site-sign-sg.tiktokcdn.com
Server:    10.64.0.1
Address 1: 10.64.0.1

Name:      p16-ad-site-sign-sg.tiktokcdn.com
Address 1: 23.46.30.62 a23-46-30-62.deploy.static.akamaitechnologies.com
Address 2: 23.46.30.82 a23-46-30-82.deploy.static.akamaitechnologies.com