If you have already tested each band for the best channel to use for best overall throughput and lowest latency for the required coverage area(s), then you need to consider better optimizing the location of the router and it's antennae, next.
The router should be at least 10' above (actual) ground level (not just 'floor' level).
The router should have at least 3' clearance all around from the walls and ceiling (of course, most homes will have to compromise here, particularly with the ceiling height clearance (the height above ground level is more important).
The antennae are usually best orientated straight up for best coverage on a single floor. Tip one or more of them slightly (very slightly) to be more perpendicular with targeted devices on a floor other than the one the router is installed in.
The router should not be in a closet, cabinet or other restrictive space (particularly metal structures, nearby studs and/or beams).
Test small movements (left, right, up, down) of the router within it's current location. You may be very surprised how a few inches or a few feet can impact the range.
If all of the above doesn't help to a satisfying degree, then the only real solution is to locate the router nearer to the center of the area(s) needing coverage.
Or, add an AP (wired) as pege63 suggests.
Trying new antennae can possibly help. But that is a trial and error method that involves blindly testing products shipped from (mostly) somewhere on the 'net.
https://www.amazon.ca/dp/B00DMJI9TA/?tag=smallncom-20
Some people on this forum have had good results with the Super Power Supply products. Others, not so much.
See what you can do (test different channels, move/locate the router better) with what you have. You can always try the antennae next (just don't think it will solve the issues completely that you face now).