Streaming from Amazon - low bandwidth

I made a call to tech support because my internet was slow (especially at night).  But the kicker was Amazon started giving me errors about low bandwidth.  There was a problem but it appeared to be in the modem and that was replaced.  The prognosis was if I still get low bandwidth errors I needed to upgrade my old router to dual band 5ghz.  So I did that and my internet seems to be much better but I still get the low bandwidth error.  But I never had any problems with Netflix.  Not then and not now.  What's next?