networking: DNS settings ignored

For several months, I have been successfully connected to the net via a static IP with my machine that is running ubuntu (16.04.3). I recently had to reinstall the OS, and now on the same machine, the system is ignoring my DNS settings. The DNS server hasn't changed, nor has the machine's static IP address changed. Furthermore, I can ping the DNS IP addresses with no problem from the ubuntu device. As a double check, other machines that I own and which reside on the same network have no trouble with DNS using the same addresses.

For the purpose of this discussion, assume the following hypothetical settings:

Hostname:    host-o-rama-bama.com
Static IP:   10.20.30.40
Gateway:     10.20.30.1
Netmask:     255.255.255.0
DNS server:  100.110.120.130
DNS server:  100.110.120.140

Using the Ubuntu Network Manager, I configured these settings as follows:

General
* Automatically connect to this network when it is available: Yes
* All users may connect to this network: Yes
Ethernet
* Device: enp2s0f1
* Wake on LAN: Default
802.1x security
(None)
DCB
(None)
IPV4 Settings
* Method: Manual
* Address:  10.20.30.40
* Netmask:  255.255.255.0
* Gateway:  10.20.30.1
* DNS Servers: 100.110.120.130,100.110.120.140
* Search Domains: host-o-rama-bama.com
IPV6 Settings
* Method: Ignore

After starting the network ...

# ifconfig
enp2s0f1  Link encap:Ethernet  HWaddr 80:fa:5b:4c:02:07  
          inet addr:10.20.30.40  Bcast:10.20.30.255  Mask:255.255.255.0
          inet6 addr: fe80::82fa:5bff:fe4c:207/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:63252 errors:0 dropped:0 overruns:0 frame:0
          TX packets:40966 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:53814860 (53.8 MB)  TX bytes:5441842 (5.4 MB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:7029 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7029 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:650606 (650.6 KB)  TX bytes:650606 (650.6 KB)

At this point, I can ping each of my DNS servers via their IP addresses.

Furthermore, remote sites can ping my ubuntu host both by its static IP address and also by the name host-o-rama-bama.com, which is already set up on the DNS servers. I can even ssh into my host both via its IP address and its DNS name.

However, I cannot see any domain names from the ubuntu machine, but , I can indeed access anywhere I want on the net via ssh, telnet, http, ping, etc. as long as I use the IP address.

In /etc/resolvconf/resolv.conf.d/base, I put the following, and then I ran resolvconf -u ...

domain host-o-rama-bama.com
search host-o-rama-bama.com
nameserver 100.110.120.130
nameserver 100.110.120.140

However, after running it, the DNS is still not working.

I put the following into /etc/network/interfaces ...

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

auto enp2s0f1
iface enp2s0f1 inet static
address 10.20.30.40
netmask 255.255.255.0
gateway 10.20.30.1
dns-nameserver 100.110.120.130
dns-nameserver 100.110.120.140
dns-search host-o-rama-bama.com

I then did the following:

# ifdown enp2s0f1
# ifup enp2s0f1

Nothing changed. The DNS is still not working.

I then commented out everything in /etc/network/interfaces from the auto enp2s0f1 line to the bottom of the file and then did this ...

# ifdown -a
# ifup -a

Still no DNS.

I then tried this ...

# /etc/init.d/network-manager stop
# /etc/init.d/network-manager start

The DNS is still not working.

Then, I did this:

# service networking restart

... and still no DNS.

What am I missing?

Thank you in advance.