I used Roam Mobility recently (just returned from July visit) and was happy with the service. I purchased the SIM Card at a retail store in Canada AND then purchased the monthly plan $39.95 (+taxes) online as it was more economical than the per day rate for our trip.
You will need an unlocked phone which you stated that you already have.
I only used the data to access Disney's Wait Time App (NO Maxpass use) and the 1GB of 4G LTE data was more than sufficient for my needs. There's unlimited 2G data once you reach your 1 GB limit of 4G LTE but since I was still using the 1 GB of 4G LTE data, I can't comment on the 2G data speeds.
There's a monthly option for 4GB of 4G LTE data (throttled to unlimited 2G speeds once you reach the 4GB limit) if you think you might need more 4G LTE data.
Again, there's also per day rates if you are visiting USA for a short visit.
Never really lost service unless for rides like Soaring or inside some buildings. If service was lost outside it was quickly rectified by turning my data on and off.
I liked that I had unlimited global text (texted a lot) and unlimited calls to USA & Canada (I understand some plans only cover calls to USA phone number and still charge for calls to Canada).
Here's the link if you want more info:
https://roammobility.com/plans
Note: click on the Monthly Tab if want monthly info
Good luck!