You're right to be disappointed. All the US national carriers have oversold the current state of 5G, T-Mobile included. Their widely advertised 5G network also relies mostly on low-band 600Mhz frequency right now. The range is good, it works indoors, but speed is not much different than 4G. The key for T-Mobile is they have started to launch mid-band 5G in the 2.5Ghz band using spectrum that they got from Sprint, which had a ton of it. Verizon and AT&T have very little mid-band available to them. Mid-band 5G is not as fast as the ultra high-band mmWave 5G but significantly faster than low-band 5G or 4G LTE. Mid-band frequency is what almost all other countries besides the USA are using for their 5G, the US didn't because that spectrum was already being used. However, the mid-band spectrum is being cleared and will be auctioned next year - the assumption is Verizon and AT&T will be very active in those auctions (they're also petitioning the FCC to prevent T-Mobile from acquiring more spectrum on the grounds that T-Mobile already has too much).