@Gertrud 

Hi, welcome to our community.

Here are some suggestions for your reference:

1.Reboot the router and your iPhone for a try.

2. Check the wireless password,fForget the wifi network and reconnect it on your phone.

3. Set different wifi names for 2.4GHz and 5GHz. Change the encryption as WPA2-PSK(AES) . Connect your phone to 2.4GHz/5GHz band seperatly for a try.

4. Check parental control or MAC filtering on the router

5.Update iOS to latest.

6.Change DNS on the DSL modem router.

If the steps above does not help, it is suggested to contact Apple support for take further research, because your other devices can connect to the router properly.

Thank you and best regards.