you will need to have an active cellular plan on the watch, to use cellular.
the phone's sim has nothing to do with it, unless your watch is close and connected via bluetooth.
if there is wifi where you're going, the watch can probably join that.
The watch can only join 2.4GHz wifi (should be in most places). It can't join wifi networks where you have to use a web page to log in like a lot of public wifi has, even if it's just clicking "agree" to the terms. But anything with a normal login should work.
all features work the same, as long as you have a connection, no matter what kind.
you'll be able to make calls (regular or FaceTime) and send messages (iMessage or SMS) with the phone at home. I've tired in the past with the phone powered off, and I'm still able to make regular calls and send SMS. (might be carrier dependent, I'm T-mobile US)
Some things, where there watch and phone directly interact, like seeing the watch's battery level on the phone, "pinging" the phone from the watch, and Do Not Disturb syncing between the watch and phone require a bluetooth connection.
one thing to keep in mind, different radios use different amounts of power
and your watch will always use the lowest power one available.
A watch connected via bluetooth to a phone all day will have more battery left at the end of the day, than one that was on cellular for several hours. And it's possible using a "full time" cellular connection will cause you to not have enough battery to make it though the day. Especially if you're in a low signal area, as the watch has to "talk" louder to be heard, using more power.
