Cannot get a connection to WRT-1900ACS

I was trying to setup DSN-over-TLS by installing unbound, using this as a rough guide: Quad9 DNS encryption

After installing unbound & unbound-control and removing dnsmasq, I set the following and rebooted and now I cannot get any sort of connection to the router. Is there any way to fix this?

Here are settings I did before rebooting to no internet connection:
vi /etc/config/unbound
option dhcp_link 'odhcpd'
option validator '1'

vi /etc/unbound/unbound_ext.conf
name: "."
forward-ssl-upstream: yes

If all else fails

Following those directions, once I'm booted to the alternate firmware, what do I do then? Is there a technique for resetting the settings of the MAIN firmware or flashing the main firmware? Anything specific I should know about this?

Figure out what you did wrong, flash again and don't repeat. In making changes like this I would tend towards staying connected via ssh, make changes and restart things via /etc/init.d/unbound restart, or whatever, saving reboot until I am sure of setup correctness. But at least you have a way out of a catastrophic FU by having the dual partition layout.

"flash again."

Is this a normal flash, or is there something specific I have to do to make sure it goes to the right place or something, with me being in an alternate firmware place?

There is nothing special about the two partitions, other than one is marked current, i.e. the one that will be booted; and this can be overridden by a uboot environment variable.

useful script.

anomeome, my deepest thanks to you. Thank you so much for posting and helping me.