stormfiber ONU with Wifi is just okish ...not too great specially where high too much signal interference is
your best bet is to change channel settings from router
download app wifi analizer and look for traffic on channels....1 , 6 and 11... out of these 3 select the one with low no of usage
if this still dont solve then simply buy a TPLINK router (3 anteena )..connect it via ethernet to main router and make it access point......you need to get it setup by someone if you dont know how to....else couple of youtube tutorials can make it easy for you
avoid repeaters or extenders as their working is limited to this only...they are easy to setup but had limited usage
router can be used for multiple purpose and had better wifi stability as per my experience