OK, I've figured it out thanks to the video attached below; the new GUI is shown from minute 8 onwards.
Basically, when you just touch and release the "Settings" icon, the device asks you for the password you set during the initial boot after the FW upgrade. This menu is the classic menu from where you can set the ringing sounds, volume, brightness etc.
To enter the menu where you can set the SIP server parameters and also the VTO's IP, one has to touch and hold the "Settings" icon for about 5-8 seconds. A password prompt will pop up; here you have to enter the same password you use for the other menu I described above (the one that the device asked for after the FW update), and not the Dahua standard "888888" or "002236"!
So to sum up, my VTH1550C now works flawlessly with General_VTH151X_Eng_SIP_V4.300.0000000.5.R.20181024. Well, sort of flawlessly... Today I noticed that the time is once again GMT and not my time zone 😡 But the automatic time update can be turned off.
Two interesting things, both related to this second menu you get after holding the Settings icon for 8 seconds. One, there is the option to turn on SSH. And second, if you have forgotten your password, there is the option on the password prompt to send you an email how to reset it (according to the text, it will send some data from the VTH to Dahua (e.g. serial number)). I have not tried it.
Video showing the new VTH GUI>