You do have the option to enable unlimited iCloud Photo Library updates on iPhone and iPad while on cellular, here’s how.
Keep Your Photos and Videos Backed Up All the Time by Enabling Unlimited iCloud Photo Library Updates Over Cellular
Whenever you connect your iPhone and iPad to Wi-Fi, that’s the time your device saves everything to iCloud. If it did so over cellular, you would end your entire cellular data plan for the month way sooner.
However, if you have a truly unlimited plan, you do have the option to enable unlimited iCloud Photo Library updates over cellular. This means every time you hit that shutter button, no matter where you are, and over cellular, your photos and videos will be instantly backed up to iCloud.
Here’s what you have to do in order to enable this feature:
Step 1. Launch the Settings app.
Step 2. Scroll all the way down and tap on Apps.
Step 3. Find Photos and tap on it to open.
Step 4. Here you’ll see an option for Cellular Data. Find it and open it.

Step 5. Now enable the Unlimited Updates toggle switch. Also make sure Cellular Data is enabled.

Apart from excess data usage, you will also experience slightly worse battery life on your iPhone and iPad. This is to be expected when there is more cellular data usage on the go, especially in areas where cellular signals aren’t that great.
But hey, you have an unlimited data plan and can’t find a good reason to end it every single month. You now know what to do.