Use Speedport Entry via LAN as access point?

I live in a dorm and have in my room only a Lan connection (no telephone jack). Everything is set up so far (Internet works if I connect my laptop with a network cable).

But I would like to connect other devices to the Internet, or have a Wi-Fi access. From my parents I got an old Speedport Entry Telekom, which I would like to use as an access point (?). After researching for a long time, I understand that it should be possible, but somehow it does not work and I do not know where the problem lies.

I'll describe the process in as much detail as possible:

Gray DSL connection of the Speedport Entry with network cable plugged into the LAN and to the mains
All 5 LEDs (Power, DSL, Online, WLAN, Telephony) flash a few times, then only power and WLAN light up
With my laptop I can connect to the router, signal strength: excellent but "no internet access"
So first tried to reconfigure → pressed with a thin object in the "reset" hole
On the laptop, the configuration page via the browser with speedport.ip called and logged in with device password
Configuration - Network - Internet access: with DSL connection "Other provider" (instead of Telekom) selected; For Internet access data: user name, password, own IP address and DNS server entered, check "Always online". (What is "MTU"?)
Configuration - Network - LAN: with IP address gateway last digit 1 replaced by 2 as well as DHCP function disabled (read that this step is probably necessary to use the Speedport Entry as an access point)
If I go to Save on network / Internet, a small window appears with the message "The Internet connection check failed. The DSL connection is disturbed Check if the DSL cable is properly connected." […] "
I was wondering if it was because I need a phone line, but I also read that it should be enough to plug it into the LAN.

I'm sorry, I know because I'm not out, I do not know if I made mistakes in the configuration or the problem is the Speedport Entry itself. I do not know all the technical terms when I tried to describe the problem to my brother on the phone, he said, I googled in connection with the keywords distributor, hub and amplifier - but I did not find anything helpful, Maybe there was a misunderstanding and we talked somehow or so…

I sincerely hope someone here has an idea of what I can / should do.

You should do the following steps:

The Speedport Entry is connected to the LAN port in your room with a network cable. At the Speedport Entry, the cable is not plugged into the DSL socket, but into the LAN socket.

At the Speedport Entry itself, DHCP must be disabled and another local IP address must be set. Important: This IP address must be in the area specified by the main router and must not already be occupied by other devices.

Example:

Speedport: 192.168.2.X

Fritz box: 192.168.178.X