How to accommodate large text within multicolumn

I’m trying to design a table for comparison between 3 subjects. Wherever subject 1 and 2 share the same features, I want to merge their columns with multicolumn (as Col. Ref.1 and Col. Ref.2 on Line-1 in the pic). It works fine as long as the text size is small. When the text size is large, however, the table exceeds the page limit, i.e., line break doesn’t occur. Any help?

The MWC and its resulting PDF is attached.

documentclass[journal]{IEEEtran}
usepackage{booktabs}

begin{document}
    
begin{table*}[tp]
    begin{tabular}{p{1cm}p{5.2cm}p{5.7cm}p{5cm}}
        toprule
        Line no. &
        multicolumn{1}{c}{Ref. 1}
         &
        multicolumn{1}{c}{Ref. 2} &
        multicolumn{1}{c}{The proposed}
        toprule
    1 & multicolumn{2}{c}{PUs cannot access the R-band directly.  }& PUs can directly access the R-band.  
    hline
    2 & multicolumn{2}{c}{The number of reserved channels dynamically changes. However, no more than a fixed number of channels can be reserved, which is kept fixed and static (i.e., half of the number of available channels in the network) } & The number of reserved channels dynamically changes. Moreover, any number of channels can be reserved depending on the number of active SUtextsubscript{H} in NB.  hline
end{tabular}
end{table*}
end{document}