It's certainly not easy to keep up with all the 5G breakthroughs, developments, upgrades and expansions pulled off in recent months and planned for the near future by the second-largest wireless service provider stateside, but even by T-Mobile's industry-leading standards, one particular advancement stood out back in early August.
Instead, the smartest thing to do is remove your current SIM card from your phone and look for the "R15" text in pink. If you can find that, you're good to go. If not and you see the letters "TM" followed by 4 numbers, you'll need to switch to a new SIM.
But even in this case, you may not have to contact the carrier and request a fancy new SIM card after all, as said fancy card should have come in the box of your T-Mobile 5G-capable device at the time of its original purchase or shipment. So, yeah, perhaps it's a good idea to locate that box and check to see if you forgot the SIM card inside, sticking to an old one out of negligence or laziness.
In addition to a standalone 5G-compatible SIM card, of course, you'll also need a handset that supports the technology, and although T-Mobile plans to "eventually" deliver software updates to "all 5G capable devices" in order to activate the feature, that hasn't happened yet.
Samsung Galaxy Note 10+
In case you're wondering, a standalone 5G connection is fundamentally different from a non-standalone (NSA) signal, operating independently of a 4G LTE network and thus promising major speed upgrades and heavily reduced latency... at some point down the line.