Diesel Bombers - New thermostat not reaching temp.

Hello y’all just bought and installed a brand new cummins tstat CUM3946849. It’s the 190 version. I only changed because the seal was leaking on the old one. The old one would get to max of 194 and stay there while driving. This new one doesn’t even reach 190 it sits right about 181 on my quadzilla gauge. Any suggestions on what to look for or what I could’ve done wrong? I refilled the rad to the max and squished the upper hose to get air bubbles out and I also filled the reservoir a little over max. I went back out the next morning and the rad is full to the brim. It’s also 30*f here but never had a problem with the old tstat getting to 194 in -20*f. The reason I changed then out was the rubber o ring around the old tstat was bad is there a way to just replace that so I can run my old tstat because this new one doesn’t seem to work.