Thats odd! Especially for the ultra. I use the base version of the S21 and I get way more (and better) signal than I did on my old phone (A51 5g). I get 4g everywhere that I have been and a fair bit of 5g too. Thats so odd that your S21 Ultra is having a problem but my S21 (base model) is going strong. Might be more that the unit u got was fault rather than the device in general.
But which mobile network was either with. Here in SN15 there are several spots where no services work. They are natural dead signal areas because of Nimby syndrome which does not accept that more masts are needed to aid signal bounce.