SUMMARY: I found my data woud sometimes be available and sometimes not, in the same places. I found a workaround was to set the preferred network type to 3G instead of LTE. Then once the data springs back into life, you can set LTE as the preferred network type.
DETAILS: I have been using Fido Roam while in the US a number of times this year. Each time I go down there, sometimes I get data and sometimes I don't. In fact, getting data can come and go at any time it seems. It's not related to where I am because it comes and goes in the same place, and the network signal looks strong at all times.
I get a connection to AT&T. It is connected all the time. Data roaming is on. I can dial voice calls. Voice calling works during the times when I can't get data and when I can. I turn off wifi for hours to see if data will come back, and it doesn't. I turn off mobile data and turn it on. I turn off data roaming and turn it on. I turn off the phone. Leave it off for hours, turn it on, still no data. I reset the wifi, mobile and Bluetooth settings, and still no data.
Then at some random point later data is back. That lovely "LTE" indicator is there.
Eventually I found a way to kick things back into life and get data, rather than wait.
The trick was to go into the network settings of the phone and set the preferred network type to 3G instead of LTE. Then once the data springs back into life, you can set LTE as the preferred network type.
On my Pixel running Android 9, these settings are under Settings -> Network & internet -> Mobile network -> Advanced -> Preferred network type. Something similar will be possible in older Android versions and on an iPhone.
Welcome to the community!
It's also possible that the surrounding AT&T cellular towers do not have the proper LTE bands/frequencies for your phone or the frequencies may have degraded too much to reach your location adequately.
@MartinS1 wrote:...In fact, getting data can come and go at any time it seems. It's not related to where I am because it comes and goes in the same place, and the network signal looks strong at all times....
I understand the network signal looks strong, but the different bands/frequencies have different characteristics. The higher frequencies -- those often used for LTE -- do not travel as far, nor penetrate as deep as the lower frequencies (see graphic here). A strong signal on the '3G' network has no bearing on LTE signal strength. **edit** A weak LTE signal could account for it coming and going as the signal might not be strong enough to maintain the connection**
Generally, Fido/Rogers customers will roam on either AT&T or T-Mobile networks as they operate UMTS (WCDMA) & LTE networks and cover most of the US (T-Mobile also has a limited GSM network). It's possible your location might be better suited on the T-Mobile network. For example, on a recent trip to New York, I found there were occasions where I needed to manually switch networks. You might consider manually switching to the T-Mobile networks to see if that allows a more stable connection to LTE.
Hope this helps