You're over complicating things... You have gone from wanting a VoIP router with built in VoIP capabilities to a corded VoIP phone, ATA and back to a router with VoIP capabilities again.
TP Link are wrong!
TP Link likely got confused and thought you wanted to connect a standard DECT phone to make VoIP calls via the router. The Gigaset 4751P is VoIP capable and all you need to do is connect it to the TP-LINK VR400 V3 router via a spare Ethernet port and it will continue to work as it did with the old router.
ATA
You don't even need one unless you want to use a corded or DECT telephone intended for use with a PSTN network with your VoIP service.
(If you do then your best option is as
@WelshPaul recommends
here.)
VoIP WiFi Router
If you want a router with VoIP capabilities built in then check out the link below:
https://www.broadbandbuyer.com/store/vo ... i-routers/
Your cheapest (and most simple) option would be to buy a TP-LINK VR400 V3 router and continue using your existing Gigaset 4751P device along side it (
IT WILL WORK 
).
