You may also need to tell us about your clients and your physical environment to help with your question(s).
If you are interested mostly in range, physically set up the router in the center of the area to be covered, at least 10' above ground level and with the antennae pointing straight up (all of them), you want at least 3' clearance all around the antennae/router in all directions (including the ceiling and side walls).
You also need to be on 20MHz channel bandwidth and test each (fixed, not auto) channel (1, 6 or 11) to see which gives you the greatest range (only one channel will be fully optimized for the radios/transmitters and the antennae/electronics your router was shipped with) in your environment.
To ensure you properly set this up; you need to use new SSID's too. The link below explains why.
https://www.snbforums.com/threads/i...-ax88u-rt-ac86u-node.60551/page-2#post-532915
While I would also recommend a full M&M Config (if not a full Nuclear Setup) to make sure you are working from a good/known state with your router, the above hints may be all that is needed (for a start) to get you the maximum range possible.
Since you have the RT-AX88U, I would not recommend any previous versions than the latest (RMerlin alpha... currently) available. With hardware this new, the changes and improvements are too much to ignore for (possibly) slightly more range.
Take care of the core requirements above first. It may be all that is needed.
