If your observations are correct, that makes two problems:
1. OpnSense cannot resolve DNS names.
2. Your former ISC DHCP does not work any more.
Obviously, the former version was not 26.7-based, but probably much earlier. There were several serious changes from, say, 25.x to 26.7., including which DHCP is preferred.
Thus, you essentially have two options:
a. Install the os-isc-dhcp plugin to restore your ISC DHCP setup - while it is EOL, this will give you local subnets again, solving point 2.
For point 1, I would first try to chec if "nslookup www.google.com 8.8.8.8" from the CLI of your OpnSense works, verifying WAN access. Then you can try to use Google DNS for OpnSense and Unbound.
b. Follow along the DNSmasq instructions from the docs, preferably on a fresh install to rule out other/older DNS or DHCP services blocking DNSmasq. If you switch DHCP, your reservations or internal DNS names will not be carried over automatically with either DNSmasq or Kea, anyway. There are external scripts to do that, but with a normal installation, it is probably better to do it manually.
I would prefer to start from scratch in your situation, because even the firewall and NAT rules have been changed. The jump from your old version to the current one is too far to justify the migration effort. I think it is easier to start fresh, maybe you can jot down or print out the relevant old rules and DHCP reservations first.
1. OpnSense cannot resolve DNS names.
2. Your former ISC DHCP does not work any more.
Obviously, the former version was not 26.7-based, but probably much earlier. There were several serious changes from, say, 25.x to 26.7., including which DHCP is preferred.
Thus, you essentially have two options:
a. Install the os-isc-dhcp plugin to restore your ISC DHCP setup - while it is EOL, this will give you local subnets again, solving point 2.
For point 1, I would first try to chec if "nslookup www.google.com 8.8.8.8" from the CLI of your OpnSense works, verifying WAN access. Then you can try to use Google DNS for OpnSense and Unbound.
b. Follow along the DNSmasq instructions from the docs, preferably on a fresh install to rule out other/older DNS or DHCP services blocking DNSmasq. If you switch DHCP, your reservations or internal DNS names will not be carried over automatically with either DNSmasq or Kea, anyway. There are external scripts to do that, but with a normal installation, it is probably better to do it manually.
I would prefer to start from scratch in your situation, because even the firewall and NAT rules have been changed. The jump from your old version to the current one is too far to justify the migration effort. I think it is easier to start fresh, maybe you can jot down or print out the relevant old rules and DHCP reservations first.
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