I'm in the process of setting up a 5g modem routing through the vigor2850n router. When the modem is connected directly to the PC (Via ethernet) the download speeds are roughly 450mb/s, upload sits in the region of 45mb/s. Routing the modem through the WAN port on the Draytek router the download speeds drop to about 70mb/s, upload speeds remain un-affected. the bandwidth isn't restricted, firewall isn't activated and I can't see anything else limiting the speed.

Any help on this topic would be greatly appreciated.

I think its simply time for a new router...there's no way that router is going to do 450Mb/s through one of it's WAN connections...especially considering it was released in late 2011 and hasn't had an update since 2017!