Lowering MTU on ES450


We have an ES450 with a Verizon GSM connection connected to a Raritan Dominian SX2 via USB. We are having in issue with Verizon only where larger packets aren’t making it. I was told by someone that the MTU needs to be lower then 1428. Is there any way of adjusting this on the ES450 or does it need to be done on the Raritan?