Question about Zyxel G.Fast XMG3927 Bridge

Unfortunately no. Standard values up to 1500 bytes and PPPoE up to 1492 bytes are supported.

MTU 1508 cannot be configured, but it works out of the box.
Source: I used it myself for a few years with Monzoon as my ISP.
Please note that not all providers can cope with this. I couldn't get it to work with iWay.