2.4 GHz Speed Issues
So my household beforehand had a WNR2000 that wasn't necessarily the most reliable with the amount of disconnection issues spreading throughout the house more in terms of the smart TVs trying to run programs like Netflix and Hulu so I decided to upgrade to a R6250 which seemingly solved the problem for awhile but now I'm unsure of whether to be suspicious of my modem or my ISP by this point due to the new problems I'm gauging with these seperate channels.
In terms of the 5 GHz things seem to run fine:
Though due to it's limited range and my bedroom being far enough away, I don't last too long on it before I'm defaulted to the normal 2.4 GHz which I'd be fine with due to it normally actually giving me 20 ~ 25 d/l and about the same towards it's upload but as of these last 2 days I've been getting nerfed down to this:
I've done various resets and reboots with the firmware being the latest version and even switching from the 5 GHz down to the 2.4 GHz brings it's standard back for as much as a half a minute before I'm dragged back down to the result above. I'm completely out of ideas.
Re: 2.4 GHz Speed Issues
For conducting speed tests, a test should first be done using a wired connection to rule out if you have an ISP issue or, not. If you suspect it is the router, then conduct a wired speed test directly connected to the ISP modem. This will confirm or not, whether you are actually getting the speed you pay for..
Wireless - Although 5ghz provides better throughput, it does not have the range of of 2.4ghz, as you have noticed. Wireless signal is also dependant on environmental interference (walls, microwaves, building age, construction, etc). Try getting the 6250 more central to your wifi user area, and, elevated if possible. I believe the antenna's are on the back of the 6250, so you could try turning the 6250 to see if signal increases.
Ensure WMM is enabled on your router and wireless client. Check to see that your client wireless drivers are up to date.
With respect to the ISP modem, is it a true modem, or is it a router modem combo with wifi enabled. If it's a router / modem combo, is it in bridged mode?
If the ISP modem is a router / modem combo, you may want to consider using the ISP provided device as the base router, and configure the R6250 as an Access Point. Then connect 6250 to the ISP device, as it is now, allowing you to move the 6250 farther into your home with a longer ethernet cable.
Ensure that any ethernet cables you are using are at least CAT5e, or better. Hope some of this helps.