couple of things,
firstly before dumping your existing router it looks from the user guide that it is band selectable (2.4GHz or 5GHz, but not both) so if it is currently set to 5GHz you may want to try setting it to 2.4GHz as the signal from 2.4GHz travels further which may help your situation. The 2.4GHz band is often more congested with neighbouring networks as there are fewer channels avail , so it's a case of swings and roundabouts.
secondly, range extenders might help as John's suggested, but you need to keep in mind that they do so by receiving then rebroadcasting the data - effectively halving the possible throughput. Might not be an issue in your situation but I've seen people get caught out thinking they are a magic bullet and then are dissapointed by performance.
I did a Wifi survey before christmas on a house where a range extender was installed just on the fringe of existing signal, and the area of improvement was a circle about 5' diameter. (I do networking for a living, for my sins)
Can you run an ethernet cable from where the existing router is to the other end of the house? if so then you might be able to get away with a Wifi access point. If not then another thing that might be considered is a powerline kit, which relies on transmitting the data using your household mains cabling and then back out to a Wifi unit. This type of setup only works if the mains cabling is all on the same source fusebox.
some fairly old canon camera stuff, canon lenses, Manfrotto "thingy", 1D MK II converted for IR, and now an M5
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