I have the exact same issue in Norway. I usually live in Romania, and I have an iPhone 6S Plus, factory unlocked. Bought right here in Romania. Used it in Bulgaria, also used it in Thailand when in vacation. Phone works just fine.
Started working in Norway about a year and a half ago, bought a Telia SIM, with Roam like Home option so I can call directly in Norway from most european contries at no cost.
SIM card works perfect from anywhere, aside Norway. Used it in Thailand, also using it in Romania. Works just fine.
Now when I am in Norway, phone gets No Service with any type of SIM. Tried using my Romanian Orange SIM, still same thing.
Now, I tried these two SIM cards on an Galaxy S7 Edge, as well as a Lenovo Vibe C, just to be sure they work, and surprisingly, they work flawlessly.
Called Telia support, asking if my IMEI is somehow blocked or anything, guy said he can't see anything.
I have a A1687 model, bought as unlocked, did not unlock it afterwards or anything.
So the phone works with the norwegian SIM card everywhere else but not in Norway.
I guess there is something with the carriers in norway, regarding foreign apple devices. Because, at the same time, the same iPhone, same model, bought from Norway, works just fine with the same SIM.
Tried everything, resetting, full reset, network reset, got the SIM in and out.. nothing.
Also tried the same SIM on a iPhone 5S, also from Romania, exactly the same thing. Once, I managed to get 4G, but the internet didn't actually work... only shown as connected.
Note that both phones work fine with the norwegian SIM in Romania, Bulgaria and Thailand / Malaysia.
If you have any followups, let me know please.