Check the CONSOLxx member in the SYS1.PARMLIB what UNIT is specified..
Possibly it says UNIT(3270-2) ...
Change it to UNIT(3270-X) and possibly then connect as model 2 and Model 4 screens sizes should be possible..
Depending on the Terminal specification of the Session in AP..
Also check the AREA and SEG parameters in the CONSOLxx member.. Possibly these need a change because of the sizing..
The actual issue is the SEG parameter...
SEG(19) is according IBM a Model 2 SEG setting..
See this IBM page:
Changing this to SEG(39) will make a Model 4 terminal