My S10+ switches to 3G data during phone calls... 4G data is unavailable during phone calls. The 3G data is very slow. My S8 on the same carrier does not have this problem.
I would recommend reaching out to your carrier for further assistance.
Per the Samsung rep my wife and I spoke with this is part of the reception problems the s10 phones are having straight out of the box....nothing to do with the carrier at all.