So closing the thread, many thanks again to all of you for your good advice.
A colleague of mine has got Sky ADSL, a Draytek Vigor 120 in bridge mode and a Netgear R7000 and guess what, it works, same settings like mine...
So no more time spending on this. Also, in this forum there is anothe user that was trying to do the same thing with an Huawei modem, but no chance that was working in bridge mode, only as a router, exactly like me and he ended up buying a Draytek Vigor 120, which apparently, along with its bigger brother Vigor 130, are the only capable to deal with Sky ADSL in bridge mode...
P.S. all the stories about DHCP 60 & 61 seem to be related to the fiber/VDSL connectivity only.