I have managed to turn my Raspberry Pi into an access point following these instructions:

Access point as WiFi router/repeater with additional WiFi-dongle

However, everything connects to the access point except my android devices (A Samsung Galaxy Note 8 and a Samsung Galaxy S5).

A similar problem is presented in this question:

Android cannot connect to Pi's wlan

However, the answer given there does not apply to me since I have the file /etc/systemd/network/08-wlan0.network exactly as in the first link. (hence with DHCP server activated):

[Match] Name=wlan0 [Network] Address=192.168.4.1/24 IPForward=yes DHCPServer=yes [DHCPServer] DNS=84.200.69.80 1.1.1.1 

How can I fix it?

Further informations. Both my devices can see the network. The Galaxy Note 8 keeps trying to connect, when I insert the password it says "Connecting..." under the SSID for approximately 3 seconds stops showing the message for 2 seconds and then keeps trying to connect. Almost the same with the Galaxy S5 with the difference that it also writes "Obtaining IP address" before stopping and then keeps retrying.