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 is not connecting 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):