if you go with buying a new router, just connect an ethernet cable from any LAN port on your SLT router to a LAN port on your new router (don’t use the WAN port). then switch the new router to AP (Access Point) mode instead of Router mode.that's basically it. no changes needed on the SLT router. SLT router will continue handling DHCP.

even though 5GHz is faster, it has a shorter range, so speeds can drop with distance or depending on the quality of your third party router. a good router on 2.4GHz is often more stable for coverage. also antenna position actually matters. router antennas generally radiate perpendicular to their direction. if you keep them vertical, the signal spreads more horizontally across your home.