Hah, I also got intrigued and have a drive over there at lunchtime and came to the same conclusion. Take a look at cellmapper at that site now. Pretty conclusive from the sector polygons.
I went with my Pixel 2 XL (non-VoLTE) and a Three (MNO) data reward SIM (not Smarty MVNO) and I also experienced the same issue with 4G connecting but getting no data throughput. Unfortunately I was unable to stay too long enough to swap my SIM over to my pixel 5 to see if it was indeed an issue just for non-VoLTE devices or not, but it does indicate that the issue isn't a Smarty issue.
I did also not see any 5G antennas mounted as I drove past, I don't suppose you happen to grab a photo of the antennas?