Is this possible

[1sec09ret]

I am going to buy a D7800-100uks ADSL Modem Router could I use the old router DGND3300(B) AS A extender ..I have a ethernet cable on a cattinery wire down to my hut 55 ft away.at the bottom of the garden.I have power down there as well

[antinode]

> [...] could I use the old router DGND3300(B) AS A extender ..I have a
> ethernet cable [...]

   "(B)"?  You should be able to use a DGND3300[plain/v1|v2] as a
wireless access point (WAP).  Many newer models have a one-step option
for WAP mode, but the DGND3300 User Manuals suggest that these models do
not.   But, almost any router can be configured as a WAP.  One
step-by-step procedure for such (less accommodating) routers should be
here:

      https://community.netgear.com/t5/x/x/m-p/1463500

That's written for a C6300, but the steps are about the same for any
other router (any make/model) which lacks a one-step WAP option,
including a DGND3300[plain/v1|v2].