I would start with the ISP service lines, have them make sure that they are in good working condition. Check for any t.v. line splitters as these can introduce noise on the line before the modem and reduce the signal going to the modem. Dedicated cable line to the modem is preferred. I might ask for a different or another ISP modem just in case, unless they can come verify that your signal and ISP modem and signals are working well at your place.
After that, I recommend using CAT6 cable between the ISP modem and router. I use a 1 footer between mine. Less travel distance between the modem and router. However make it sensible for you as not everyone can keep the modem and router close together.
Use CAT6 Cables from the router to all wired LAN devices as needed. Can be any length. Swap out the old ones to be sure. Sometimes the in box cables are not mfr'd well.
Disable any 3rd party firewall and anti virus programs temporarily while testing, configuring the wired speed testing the router. Turn OFF ALL other devices while speed testing as they can effect performance. Graduate by turning on other devcies and test.
Ensure any wireless or wired adapter that use drivers are up to date. If you have doubts about a wired adapter in a PC, try a different PC or different adapter to test. Adapters can go bad.
Factory reset the DIR-655 and set it up from scratch, use default settings for most of it accept for the wireless, set up SSID and PWs. Test with SPI ON and OFF. Change NAT settings to Endpoint Independent. If your ISP modem supports 1000Mb LAN speeds, set WAN port speed to 1000Mb instead of Auto. Turn off options that you don't normally need, like WISH, WPS.
Keep us posted...