Welcome to my world Valkyrii! I have the same issues, I'm sure software companies think we all live in major cities with constant internet service and you-beaut wi-fi when ever you want it
.
Ubi eased the DRM requirements of SH5 some time ago so you need only have a connection when you launch the game but once running you no longer (apparently) need a constant connection (all though I just can't make that work).
It sounds odd that a SH5 DVD should require Steam activation as Ubi choose to introduce their own DRM system called Uplay. On my end to activate the disk you had to create a Uplay account and jump through all the hoops, but Steam was never involved.
What country are you in?