I discovered that on the 2 different networks I operate, 188.8.131.52 does many times not work on our Galaxy S9 mobiles when using Chrome, Edge and FF.
Added 184.108.40.206 to the FW as default DNS to use in DHCP
Windows 10 PC work without having problems
GalaxyS9 phones do most of the time have the issue that web sites can’t be resolved.
If WLAN is disabled on the phone and the mobile network is used, resolution works immediately.
On the other hand, if the mobile connections is used and one.one.one.one is manually added to the phone, the same issues happen.
Same happens in my other network where salt.ch is the ISP.
Sites I regularly test: