Maybe, but then every router is using a different wording for this process. Or tell my how I can find an equivalent in a Fritzbox 7590 router.
And what is with all my other wlan devices? Do I have to reconfigure them all to this new fixed IP address?
But please just ignore me if you are short of time. I'm only asking out of curiosity, as port forwarding makes no sense in my case anyway.
...and that's what I wanted to ask: why do you not use port forwarding? That's likely academic and OT for this thread, so you need not answer it, of course.
As far as the specific router you mentioned: the Fritzbox 7590, I can't say that I'm familiar with its config interface (no surprise there since there are a huge number of routers out there with some great- and some terrible- config interfaces). I wish that I could help with that specifically.
*
You'll see terms such as "
DHCP reservation." Mine is called "
Manually Assigned IP Address around the DHCP List" (don't ask me why; that's awful!) On a Trendnet router, I have seen it called (much more predictably) "
DHCP Reservations List."
This will almost always (at least in sensible cases) be found in the router's
LAN settings.
My advice about making this easiest is: Look at the CLIENT LIST in your router's config interface. That is commonly a table-view showing all of the computers & devices currently connected to the router's LAN (via both ethernet or WIfi). Usually you'll be able to pick out your computer by its name on the network or by its MAC address and in many cases there will be a button right there to reserve that address.
Even if that's not the case, the interfaces will have a pretty simple table-view for DHCP reservation that has a few fields for you to provide info such as:
1) a host name (just a name of your choosing for the computer or device - not functionally imperative)
2) the device's MAC (pretty easy to find using the "ipconfig /all" command in Windows) or obvious based on the few devices shown in your CLIENT LIST.
3) IP address you wish to reserve for the device
4) an "enable" checkbox for the reservation.
You may not see ALL of those fields in your case.
Some routers benefit from a restart even after you've supposedly applied and saved these settings. YMMV.
When you say "wlan" are you referring to wireless (Wifi) access? In short, the answer to your question depends upon whether you want all of those devices to be able to use port-forwarding. If not, there's nothing more convenient than allowing each device and the router to negotiate and get their IP addresses sorted out
automatically! This is called DHCP. It's generally a wonderful thing for most of us! Myself included!

Automatic is beautiful!
To be honest, IP reservation is one of the easier tasks to do and it's useful beyond a step in the port-forwarding process. If you have a network-connnected printer, for example, this is a prime example of how handy it is for a device to
always have the same IP address on your network. Doesn't it make you crazy when WIndows can't find your printer? Reserve it, I say. Have your router make your life easier!
If you're careful not to make changes, it can be very interesting and educational to look around in your router's config interface.
* You might elect to look at portforwarding.com. they have a pretty big list of routers in their example-pool. If the Fritzbox is there, you may see instructions for doing this specifically for that router.
EDIT:
Surprise: they do!
These are our open port guides for the FRITZ BOX listed by model.
portforward.com
There's a model 7490 there. I know it's not the same, but if you look and compare with your 7590, you may well find that they're essentially identical or close. These interfaces tend not to change drastically.