From MediaTek: "How does MediaTek 5G Smart Link technology work?"
MediaTek is working to mitigate the effect of signal blockages from everyday objects that get between you and the cell tower, improving wireless connection performance and reliability. By dynamically switching between mmWave and sub-6GHz radio (in this case, 4G LTE), performance can be drastically improved versus only using mmWave alone. See how it works in the video below:
Source: https://www.mediatek.com/blog/video-how-does-mediatek-5g-smart-link-technology-work