I stumbled across this whilst researching the same issue - looks like I am doing the same thing, passing wifi credentials to the esp. Unfortunately, there may not be a simple solution, if you see the link below. IT appears that setting the MTU works only with writebytes, but not writestrings... In the past, when sending a bunch of parameters, I have sent them in several shorter strings, and will probably do the same here. But do see post#12 of the link here for the complete solution if you're happy to writebytes..
https://community.appinventor.mit.edu/t/solved-cant-send-payload-23bytes-mtu-setted-to-128bytes/48330/11