I have a Three SIM card in my iPhone, and a separate Three 4G MIFI device with it's own SIM card also.....
I've a monthly 30GB, on my pay-monthly SIM in the iPhone, whereas the MIFI has a pre-loaded SIM with 24GB, which is valid for 24 months.
I cannot create a hotspot, with my iPhone whilst roaming, this "data use type" is not allowed, and restricted by the carrier (Three.) Video streaming, is also restricted whilst roaming.... However, I can, and do, use the personal hotspot feature daily whilst on home-network, in the UK. To use this feature whilst roaming, you need to purchased a "data passport" from Three, which is £5 per 24-hours. I've purchased this previously, only to find it doesn't work.... After several calls to Three Customer Services, they refunded the £5, and advised they were aware of a current issue - I haven't tried it again since.
Due to the above issues, I've now reverted back to using the MIFI device whilst roaming.... I had intended to decommission the MIFI device, after getting a 30GB per month allowance on my iPhone, but it turns out it'll survive to live another few trips....