Unfortunately they are both USB-dongle-based, so you can't do anything to increase signal range.
Try using a short (1 to 3 foot) USB extension cable with the dongle. I had the same problem with a Logitech K400.
In my experience, keeping at a distance the USB3 peripherals from the Pi4's ports gives noticeable benefits. A 3ft extension cable is probably safer than a 1ft one.
Thanks for all the answers. Is there any possible (other) way except using an USB extension cable to a wireless operation of my RPI 4 at a 6 feet distance?
Get a Bluetooth adapter with an antenna and a Bluetooth keyboard. This should provide best signal range.
Thanks for all the answers. Is there any possible (other) way except using an USB extension cable to a wireless operation of my RPI 4 at a 6 feet distance?
Not unless you want to replace your keyboard. The extension cable is the cheapest way for a quick fix. My problem was with an Intel NUC. Before installing the USB extension cable, my max distance was only a few feet. It was probably due to interference between the NUC and the USB Dongle.
... and If the Device is mounted behind a TV you won't get a propper Signal. Also the Keyboards and mices are commonly designed to be used near the PC.
Same Here with a Logitech K400r. I also use a USB extender cable with a unpowered Hub to Connect the K400 Receiver and a Bluetooth Dongle for Audio to extend the range.
Regards
Nicolas
In my experience, keeping at a distance the USB3 peripherals from the Pi4's ports gives noticeable benefits.
In my experience the "distance" of the Pi4 is a problem at all.
The WiFi of my AVR or of my Xbox nosedives if the PI is in range, I have to put it at least 2 feet away. Same problem with my wifi-sockets and and 2 2,4 GHz transmitters for Zone2 and my subwoofer. E.g., if i put the Pi directly on the X-Box, the wifi has heavy dropouts with overall10-20 KB/s instead of 100 Mbit.
Everything works again like a charme, if I remove the Pi4 from the cabinet.
I now separated The RasPi with 2 layers of tinfoil, with only a small exit for warm air et the top and put it in the corner of the cabinet. That seems to do it in principle.
Funny, the only thing with no problems is my Logitech 520 wireless keyboard that has no problems 5 meters away on the couch. 
Hey, so just upgraded to Pi 4 (9.2.6) from 3b and having the same issue, all of a sudden the range of my keyboard seems lower. It doesn't work when I'm on my couch.
I know that this makes no sense, but could the Pi 4 be interfering with the signal?? (It's a logitech keyboard with usb dongle).
So weird!
Hey, so just upgraded to Pi 4 (9.2.6) from 3b and having the same issue, all of a sudden the range of my keyboard seems lower. It doesn't work when I'm on my couch.
I know that this makes no sense, but could the Pi 4 be interfering with the signal?? (It's a logitech keyboard with usb dongle).
So weird!
Try the dongle in all available USB ports. If that doesn't help, use a short USB extension cable (1 to 3 ft) with the dongle. It really helped with my Logitech K400 keyboard setup. I use an Intel NUC, not a Pi 4 though.
