In case, both IP address are static, write your your DNS sever in /etc/resolv.conf : nameserver 'dns_IP_address'
If one IP address from DHCP, network-manager will re-write resolv.conf with data from DHCP
If the DNS from DHCP is not good, you must fix a static address one each interface and write manually resolv.conf
What is this ?????? internet access blocking port 80 and 443
Are you sur port 8000 or 8001 are open ; for user, prefer port 8080, some internet browser block no standard port ( i have had this problem with port 8000 and must move to 8080 )
For the firewall, normally all Armbian have installed IPTABLES ( it's the native linux firewall ) but it's only on command line
I never used webmin but i think it said "not installed" because there are not filtering rules
https://doxfer.webmin.com/Webmin/Linux_Firewall