How can I fix this internet lag myself, Tech will not help!

I have been experiencing very low download speeds for several months. I have been given every excuse there is, but no solutions or fix. Running UFix diagnostics is ineffective, It just suggests rebooting the gateway, which of course I've done 100s of times to date. I have done the 192.168.1.254 deal as well which also only wants to reboot and picks up nothing. Using Smart Home App same results. The stats are... I have Pace 5268AC Dual Band Router/Modem, all speed tests show the rates for my package between 6Mbps to 7Mbps download and upload between 0.7Mbps to 4.25Mbps. Now I actually only receive up to 780Kbps download which translated is only 3/4 of one Meg. My question is where could my bandwidth be going. Devices connected are really only between 3 and 5 but show 11 connected, some active some not. When I check data usage it shows all are using data at different times, but most of them are using very little, and this does not equal the data drain. I would love to find out what "Unknown" devices are??? And find out how to disable them if they are false devices. I also have Direct TV with the wireless Genies, but when I checked the data usage they only used 10Megs in a 24 hour period. So my question is where is my bandwidth going, and how can I get it back? In addition I use a WIFI extender but have had no problems from it, and it works as it should extending the signal to parts of my home which previously didn't receive a signal. Any suggestion guys and gals? Note: I have been using an ethernet connection lately but this doesn't make a difference.