MTC could either be MTS from Russia, or MTC from Namibia. Both are very different to European networks.
Russia uses bands 3, 7, and 20. The UK uses bands 1, 3, 7, and 20.
If its Namibia, their networks only use Band 3.
I feel your second hand searching may have drawn a blank. We have no way of telling what signals are in your area as you have too old a mobile You could buy a more modern 4G Android phone from a supermarket / argos, and that would show if Three 4G is available in your home. Depending on the handset there are free apps that can show what the signals are by the band number / frequency.
It depends what you really need, but I would suggest not buying any more used items where you can’t tell what country they are from.
(The texts you see on the dongle, are they specifically from Three UK, or could it be any Three network, anywhere in the world? There are a few).
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