Internet speed

I am looking for an explanation about internet speed reading on my system. I am running a Dell Optiplex system with 8 gigs ram and Intel CPU.
My provider is supplying me with a speed of 100 Mbps which I was able to read on many speed testing services. Reading from the ethernet connection, not the wireless, I was showing close to 100 downloads and the same for upload. I was then offered a further update to speed to 300 and to 1 Gbps. I do see a speed increase but cannot read past the 100 first obtained. After the update I had the provider place there own modem/router, a TP-Link, ADSL with up to 5 k capable. When he uses his Dell laptop and reads the sp3eed it shows on his meter at 900 plus Mbps.
I have the ethernet connection with cat5 into a D-Link 1 GB speed 10/100/1000 network card. I still only see the original 100 reading.
When I ask why I get no real answer just that there equipment reads it and my doesn't.
They have been a great company and provided well for several years and also have no cap on usage.
My question is just what am I missing in my search for the 1 Gbps reading. Any reasons I can't see it and they do.
Any insight would be appreciated.

Link-local IPv6 address: fe80::cd3a:8d78:21ff:dea5%25
IPv4 address: 192.168.1.121
IPv4 DNS servers: 66.38.2.240
8.8.8.8
Primary DNS suffix: nctc.com
Manufacturer: D-Link
Description: DGE-530T Gigabit Ethernet Adapter
Driver version: 8.4.907.2012
Physical address (MAC): 0C-B6-D2-B2-A9-F1

router
TP link 34b6-2