ER-X-SFP - SFP (eth5) port has link-state (LED = lit), but swconfig says link: port:5 link:down

There's a limitation in WAN/LAN routing for ER-X-SFP (link), keep it in mind. Don't expect speeds > 500Mbps.

I needed to use ethtool to fix autoneg not working:
ethtool -s eth5 speed 1000 autoneg off duplex full

Concerning your VDSL, idk.

I have moved to BPI-R3 meanwhile.