When I did what you suggested, I had no internet at all. Apparently my router is also a modem although I wasn't aware of that fact. So I'm still wondering if this router would benefit from a firmware update or I should be shopping for a newer dual band gigabit router?
it isn't a modem also ... you might need to open (administrator) cmd prompt and type "ipconfig /renew" when hooking the network cable that goes to the wan port of the router to your pc directly
alternately if it still doesn't work ...call their support to get them to assist you on testing this
or just flash the router and try it.... a couple tips
disconnect the wan cable (and any other computers on lan besides the one doing the flash) and connect the router to the pc directly (wired) for the flash ..make sure the LAN address of the router is the default address while doing the flash
do not attempt to flash the router over WiFi as that would likely result in bricking
if you flash and it is still slow then call the ISP and ask them for assistance...if they give you crap about the router ask them for assistance testing it with the pc directly attached to the ONT... if it is still slow when you do that with them it will shut them up about your router..and then hopefully they can fix it... if it isn't slow on the pc then your router is the problem.
i assume you have tried power cycling the router already?? right??