There is a difference between plan speed and network type. Even on a 4G plan you could still access the 5G network (and 3G network). How did you have your preferred network set on your phone before? And now? Also note that plan speeds are maximum, and not guaranteed. They are highly dependent on a number of variables including signal strength and network congestion.
I am on a 5G plan but I usually just force my phone onto 4G anyway, as I still get 5G speed, but I find that the network is more reliable. 4G towers tends to penetrate obstacles better than 5G, and is easier on the battery. If you were right beside a tower with little congestion I'd expect better results on 5G.
Long story short, even on a 4G plan you likely weren't consistently getting 100Mbps. Those are maximums. Both networks are capable of faster than the plans offer anyway (they use throttles).
