If the 37 MB/s is the throughput measured with a client connected to Wifi then that's your answer ; if however you're doing the test between a client connected via ethernet to one of the switch ports in either the Innbox modem or the DOR 882 LAN ports then that is indeed an issue .
To be able to help you possibly troubleshoot I would however need more information ; it would be great if you could make a schematic of your network in which you should include for every link the following info ( physical port , IP of that network interface and if possible interface status/PHY rate as in 1 Gb/s full duplex , 100 Mb/s half duplex ) .
Also include the relevant configuration pages screenshots ( Port Fw section on the Innbox modem , DHCP server & port Fw on the Dlink ).
And also make sure that QoS is not enabled on any of the devices as that is usually not powerfull enough on this type of devices to be able to handle gigabit links (it's really included there for when clients have limited Internet bandwidth , <100 Mb/s , to make sure certain clients do not saturate the link but with higher tiers of internet connectivity now like 300/500/1000 Mb/s QoS only creates bottlenecks and adds latency instead of helping)