Have you kept any backup from before the upgrade?
Also keep in mind that packages added after the flashing of the image are not carried over to the upgraded version, so you'll need to reinstall them.
Check what is running on port 80 netstat -lnp | grep 80
As I was going for several changes on the setting and as first time I worked onopenwrt ending on almost saturating the memory with tests and several extra packages, I decided to go for a full reset via serial console.
I just uploaded the 18.04 without any format of the memory.
Surprisingly the directory /etc/config hasn't been deleted which helped on restoring most of the settings but keeping some of the trouble
At this stage most of the memory has been freed but I have problem on changing lan IP settings.
I have now online access (with old user &password) but when I try to change the IP and save the conf roll back to default 192.168.1.1
I'll open a different post as things have changed from the subject of the title
Files to be preserved depend on the following:
/etc/sysupgrade.conf - customizable backup configuration.
/lib/upgrade/keep.d/* - system configurations provided by specific packages preserved by default.
opkg list-changed-conffiles - list of files derived by package manager.
You can run firstboot from the SSH or Restore Factory Defaults in Luci. No need to flash again.
At the end, after several checks and attempts I figured out it was a problem with browser cache redirecting to the wrong configuration https VS http and https/https standard 80,443 ports VS not standard ports