What glues to use for PETG?
From  forum


  Bison plastic - works great for me, only it's not "quick dry" but it's very strong, not brittle, holds PETG and PLA very strongly. 
  Buy paint stripper that is/contains methylene chloride (dichloromethane). It will solvent weld both PLA and PETG.


Or see  other forum  which recommends Eastman-910 (original brand of cyanoacrylate) or any equivalent, polyurethane glue, or even epoxy.