I have a draytek vigour 2865 running load balancing across 2 lines
1) is BT / Vodafone VDSL2 line - WAN1
2) is 1gbps community fibre line - WAN2

This firmware update to 4459_BT has been a real headache for me.

I have 2 load balanced WAN lines one of is a 'DSL' 68mbps line with vodafone / bt-openreach after much troublehsooting I have now concluded that there is a compatibility issue between the DSL modem compnent of the draytek and the DSL line that exists in the firmware of 4459_BT and vodfaone 68mbps DSL otherwise known as FTTC.
The result of this is the draytek cannot ping the dns server in my case at google 8.8.8.8 ( commonly set) to verify line quality, this then throws it into a tizz an paying no regard to the second WAN link ( which is stable).
This then causes disruption in your internet connectivity. and downtime for 40-90 seconds while it reboots and obtains a new ip address via dhcp for the line. Reboots happen many times daily at random times.

Draytek support have suggested to try the 'varient ' downloads of said firmware these are MDM0, MDM1, MDM2 upto MDM5 all in all we have 7 varients, my draytek used to work perfectly.

Not after this update am now working through the varients with different modem codes