PROBLEMS WITH NEXTDNS AND PORT 53 OPEN

Hello to everybody,

Since a few days ago, the page of nextDNS tells me that 2this device is using "vodafone Spain" (the ISP) as DNS server... But the fact is that the router has the DNS from NextDNS. 

At the same time, the port 53 is open (locally) and I do not understand why nor how as I used to scan regularly my IP and this was not happening a week ago:
 

PORT   STATE SERVICE VERSION

53/tcp open  domain  dnsmasq 2.90

I have noted that this service is the same one reported in NextDNS installation manual for routers.

I would need some help to solve the issue because I have made a big effort (thanks to my zero level knowledge) to close all the unneeded ports.

The router is connected to a physical firewall but it was already connected before this issue.

Thank you

All the best!