Variation in Router speeds with ultra fast broadband & ont

I have just had fibre ultra fast 900/500 installed which is great (slingshot), I was originally on VDSL with a Netgear D7000 V1, so switched this over to WAN only. worked great, was getting 700-800mpbs down approx 400 up. I was under the impression that the router did not determine download speed but the ont did, well my Son wanted my box for his place so decided to give him mine & purchased a Netgear R7000P, well that slowed my download & upload down to a measley 300/200, swapped back to D7000, up it came again. I returned the R7000P & decide to splash out on the netgear D8500 (even though didnt need modem side). This time my speeds were consistent 500/200 which was better but though should be faster. So borrowed my sons genuine slingshot router (with crappy wifi) & back again to 800/400. Why are there so much inconsistency with various models, is there a hidden setting effecting the compatabilty with the ont? Does it have something to do with the way the vlan tagging is setup in each box? Using same speedtest with 5 minutes of each test so no fluctuations there

Any ideas appreciated, sick of throwing money at the issue, seems to get worse with more money.

thanks