How may I assign static listen addresses to the admin webpage ?

Did you block the Webadmin via Device Access? 

You should only allow webadmin access from dedicated hosts.

Not the target IP but the source IP should be decisive.

Thanks for your reply. How can I do it ?

Within administration / device access select only trusted zones within "Local service ACL / HTTP"

... or more restrictive ... select only trusted hosts within "Local service ACL exception rule" and allow HTTP access.

but be careful ... first allow / test / forbid ... do not forbid your own access.

Thank you for your reply. I can not yet find the option for only trusted zones within "Local service ACL / HTTP". This is my interface. Where may I find it ?

HTTPS is Webadmin.

But based on your screenshot, you are using the same Zone for every Interface. So you cannot limit it based on Zones. 

Well, I have created multiple VLAN interfaces and mostly all of them have been put under the LAN zones. Has this been un-wise for me to do so ? If so, how can I now limit it without having to rely on Zones ?

You can use one Zone, but you will lack in granulate later. With Zone based concepts and VLANs in own Zones, you can define more granular firewall rules in a quicker manner. 

Then what can I do now to achieve the question in the title of this topic ? And what else can I do later on to re-gain your ‘granulate’ !?

I just happened to come across this Youtuber who is posting tutorial video clips about Sophos XG v18. This guy has a habit of going to the 'Hosts and Services' tab and create IP hosts (host groups) before creating an interface.

Here, 2 more questions have arisen from me :

1. Does this habit have any benefits for me when using Sophos XG v18 ?
2. Does this habit increase the granularity when I issue firewall & ACL policies & rules as well ?

Thank you again buddy.