A fan will help circulate the air but it's not going to lower the temperature as much as you need. You can try adding a wet towel behind the fan but not blocking air flow to help cool the air by evaporation. It can lower the temperature in that small area by as much as 10°F. This is a crude version of what is referred to as a "swamp cooler." You can Google instructions to build a better version if you are handy, there's lots of different ways to do it. It also helps raise humidity which your species probably also needs.
Some mantis species prefer it to be very hot with less humidity such as [COLOR=rgba(0,0,0,.87)]Gongylus gongylodes,[/COLOR] Blepharopsis mendica and Idolomantis diabolica. These are not typically beginner species as they require more specialized care than other mantis species but providing extra heat is part of that so you might want to do some research to see if it's something you are capable of doing in the future.