IP address change not surviving reboot

This may be an underlying Linux problem but I first of all need to run 
it past you guys and gals here as few people on Linux forums will be 
familiar with Shorewall.

We have a Shorewall firewall at the school where I volunteer, protecting 
the school network from a Raspberry Pi farm on which students 
necessarily have root privileges. I rebuilt it at home on newer hardware 
with the outside interface IP address reflecting my home 192.168 network 
instead of the school 172. network. I took it in to school today and 
attempted to reconfigure the outside interface IP.

Using the GUI (Linux Mint XFCE), I changed the outside NIC IP address, 
netmask, def g/w and DNS server. In the GUI, the outside NIC (enp2s0) 
has the label SchlNet. Shorewall IP address dependencies are 
encapsulated in /etc/shorewall/params, and I changed those.

After a reboot, the GUI shows SchlNet has lost its configured IP address 
but gained 16 alias addresses added by Shorewall for NAT rules. 
Meanwhile, a new enp2s0 has appeared with an IP address I didn't recognise.

ifconfig shows the base enp2s0 with no IP address, plus the 16 expected 
NAT addresses on enp2s0:0:15 (or 1-16 - I forget).

I tried deleting the spurious enp2s0 and reapplying the IP config to 
SchlNet, but the same happed after a reboot.

I also tried deleting SchlNet, configuring the new enp2s0 and renaming 
it SchlNet, with exactly the same result after a reboot.

shorewall stop and shorewall clear before reapplying the config made no 
improvement.

Maybe I should be using the CUI commands, but I'll need to read a man 
page or two first, and I'm not sure whether the GUI tool maintains any 
of its own data. Anyway, a bit of insight from round here would be 
appreciated.

Regards - Philip