There's not much here... >>..
Before you can use wifi on the Underground, you'll need to register for O2 wifi. You can download the O2 wifi App for iOS and Android, or register in any of our O2 wifi hotspots. Find out more about how to access wifi on the underground.
www.o2.co.uk
.... and I haven't travelled abroad for years, but even though roaming is getting cheaper or inclusive - I would guess that texting another O2 user within a mile of each other in the US is not what roaming is designed for, and could be exorbitantly expensive.
So, as B. Diddy said it's normally wise to buy a temporary local sim in the US, even though you would have a different number (you can redirect your calls but again check the costs with O2 (I use EE).
However if you're older Moto G8 is suitable (see article) you can still either text and call over data or wifi.
A texting service over data / wifi like WhatsApp should work well if you can get 4G when there's no wifi.
(set WhatsApp up before you go)
Again speak to O2 for sure, and I could be miles out of touch.