dm800se clone - limited wifi channel band width??

hi all have a dm800se built in wifi all wasworking fine
this week my bt HH3 router broke so i started using a draytek vigor 2750 router/modem

went to use my 800se didnt work - 30 secs later realised it was setup still with bt hub so scanned found new draytek wireless network and put in wpa2 key etc but still nothing.. anyway to cut long story short..

fiddled with settings in router and set it to wireless g (it was set on mixed b,g,n,) then the dreambox was working fine..
however the wifi on all other devices eg ipads,laptops etc went from 25mb down to 2mb so i changed router to a mixed g,n (channel auto) the dreambox was still working,.. network transmitting as wireless n but ipads laptops etc were still slow so i changed the channels on the router (have a choice of 13 channels ranging from channel 1 2412Mhz - channel13 2472Mhz )
i found around channel 7 the wifi was at its peak of around 24mb (my max BB connection LAN is 32mb) however then the dreambox stops working when i define a particular channel rather than auto

so if the built in wifi only conpatable on wireless g and certain channel(freq) ranges? i just cant seem to find a happy medium between working dreambox and decent wifi speed

just tried wireless g on its on with channel 7 and that works.. but when i select g,n channel 7 stops working i always thought that it would downgrade wireless n to g anyway on non wireless n compatable devises??

ok update..
i can get dreambox to work on wirelessg channel7(2442Mhz) however when i then change router to wireless G+N) channel7 the router changes fine and the dreambox remains ok.. but once the dreambox is rebooted it doesnt connect again. also the wireless speed difference between G and N is about 10mb so i get 20mb on laptop,ipad on wireless g and 30mb when on g+n, i just dont understand why the dreambox wont work on G+N?