I'd be surprised if this were a hardware issue. But if you have an Apple Store nearby and some spare time, a visit to the Genius Bar couldn't hurt. They can run a diagnostic test on the cellular RF hardware. (Make sure you have your data backed up first, because repair/replacement will mean data loss otherwise.) To help isolate the cause, it would be worthwhile to start logging whenever this happens. Then look for any patterns, like time of day, location, any accessories in use, etc. That could help sort out whether there's an environmental cause that's within your control (as opposed to a carrier issue). Are you using a VPN? When my cellular data temporarily loses internet access, my VPN is inevitably the cause.