Guide to setting up large MTU on Synology ds216play:

1. Don't.

You can fiddle around with

/etc/synoinfo.conf
/etc.defaults/synoinfo.conf
/etc/sysconfig/network-scripts/*

but you will discover that the unit is incapable of having a large MTU set.

According to Synology the "DS216play do[es] not support adjusting MTU" [ref]Personal Communication w/Dustin@synology 25 Jan 2017[/ref]