ubuntu-18.4 - multiple IP addresses via NetworkManager are failing

I'm on ubuntu-18.4 and I have 5 static IP's available to me. They are of the following form ...

a.b.c.34
a.b.c.35
a.b.c.36
a.b.c.37
a.b.c.38

I have set them up in the NetworkManager applet by specifying "Manual", and by trying to enter each of them in the IPV4 screen as follows:

Address     Netmask   Gateway
a.b.c.34         24   a.b.c.1
a.b.c.35         24   a.b.c.1
a.b.c.36         24   a.b.c.1
a.b.c.37         24   a.b.c.1
a.b.c.38         24   a.b.c.1

However, after saving the setup and then going back to the IPV4 screen, the "Gateway" values for the 2nd through 5th entries are blanked out, and after restarting my network, only the first IP address (a.b.c.34) is active.

What am I missing in order to get my machine to respond to all 5 IP addresses?

Thank you in advance.

PS: I know that the IP addresses are valid and active, because I previously was using them with my same machine that was running Ubuntu-16.x. Whatever I did on that machine, all IP's were working (but I forget the details of what I did). But after upgrading my machine to ubuntu-18.4 and trying to use NetworkManager, the 2nd through 5th IP's are not being recognized.

Follow-up ...

This is a laptop which is being used as my desktop and also as a server. And here is the requested info ...

# cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
# cat /etc/netplan/*.yaml
# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager
# ls -al /etc/systemd/network/
total 8
4 drwxr-xr-x 2 root root 4096 Apr 20  2018 ./
4 drwxr-xr-x 5 root root 4096 Apr 17 00:52 ../

Further follow-up ...

The connection is called "Hippo-Wired". I changed the high-order IP octets to "a.b.c" here, and I also anonymized the uuid, the Mac address, and the DNS entries. Note that there is only one IP address. The 2nd through 5th addresses in the applet are not showing. For some reason, the applet didn't accept them ...

# cat Hippo-Wired 
[connection]
id=Hippo-Wired
uuid=tttttttt-uuuu-uuuu-iiii-dddddddddddd
type=ethernet
autoconnect-priority=-999
permissions=
timestamp=1588347775

[ethernet]
mac-address=nn.nn.nn.nn.nn.nn
mac-address-blacklist=

[ipv4]
address1=a.b.c.34/24,a.b.c.1
dns=sss.ttt.uuu.vvv;www.xxx.yyy.zzz;
dns-search=
method=manual

[ipv6]
addr-gen-mode=stable-privacy
dns-search=
ip6-privacy=0
method=auto

Could it be that the applet will not allow more than one IP address with the same Gateway?