It's said it will add PPTP/L2TP VPN client soon.

How about the firmware of the bulbs? Try to update to the latest one.

generally speaking, they can work in that way. Just like other wireless router and we can also get it work with powerline adapter.

main Deco——Switch ---port 2\3\4 connecting to wired devices. other wireless devices are connecting wirelessly? all of them are connecting to Deco network. You can't ping wired devices from wireless...

you can have a look at the note under 20180125 to see whether it reminds no downgrade. If no, downgrade the FW to old one.

If you want the c2600 work as a pure AP, disable DHCP server on it.