What you should check first is if you have the latest application firmware and DECT module firmware on the 300Wx. The Konftel 300Wx has two different firmware that need to be checked. One application firmware and one DECT module firmware.
You will need an SD-card to upgrade the DECT module. The application firmware you can upgrade with Konftel upgrade utility but also using an SD-card.
If you already have the latest firmware and still are having problems to pair (failure on DECT pairing attempt) you can make a default base station on the IP DECT 10 from the web interface under management. This will erase all connections and reset the base to factory defaults. So you need to redo the pairing to the 300Wx.
Also make sure that you have an IP address to the IP DECT 10. The IP DECT 10 base LED should be green. The IPDECT 10 will not accept registrations if it doesnt have an IP address.