@viragomann I understand, but this was set since the beginning as I wrote in my first post about the config:
DNS Server 1: 192.168.1.14 (pfSense, I use pfSense as DNS server with pfBlockerNg)
but I tried to add it also manually before to the config by this line, but actually did not change anything:
push "dhcp-option DNS 192.168.1.14"
What was interesting, I also saw connections earlier from the phone to the pfSense IP on port 53 based on states
(Firewall > Rules > OpenVPN, then clicked traffic data in the States column)
but something was not good as the DNS server actually not responded to the queries from the phone. At the moment I have that only idea the DNS server service was not in a good condition.
Anyway, thanks a lot for your help, I really appreciate your prompt feedbacks!