I went to my local T-Mobile store and converted my iPhone 11 Pro Max from using a physical SIM card to an eSIM.  The folks behind the counter fumbled a bit, but finally got my eSIM up and running and removed my physical SIM card.  The day after that, we lost power in our home.  It was a partial power loss, but we lost the modem and therefore lost WiFi.  To my surprise, I was unable to use cellular data on my iPhone.  It was as if I didn't even have it turned on, which I did.  I tried a hard reboot, turned off WiFi and turned it back on.  No WiFi, so that didn't do anything.  Called the local T-Mobile office.  They suggested resetting the Network settings.  I did that.  Didn't work.  Called T-Mobile main line.  Talked to tech support.  They couldn't help and put in a help ticket.  It was very frustrating that I didn't have cellular when my wife's iPhone XR was fine.  Her phone could access cellular data.  The next day, I was still frustrated.  Power was back, but I still had this issue that needed to be fixed.  I was finally able to fix it using the following technique.
1. Turn off WiFi on the iPhone.
2. Do a hard reset on the iPhone.
Once the iPhone came back up, it saw the cellular data. Now I can turn off WiFi and use cellular data. Not that I want to, but if I lose WiFi, it's nice to have the cellular data. Or, if I am out and about, I'll need the cellular data.
	
		
			
		
		
	
				
			1. Turn off WiFi on the iPhone.
2. Do a hard reset on the iPhone.
Once the iPhone came back up, it saw the cellular data. Now I can turn off WiFi and use cellular data. Not that I want to, but if I lose WiFi, it's nice to have the cellular data. Or, if I am out and about, I'll need the cellular data.