If I use the new 3100 router or 1510 access point I get great results on 2.4 and 5g

If I use the 1200 router and/or the 1410 access point I only get an average of 20mbps on 2.4.  5g is fine..

this is with 100 mbps.  the 3100 and or the 1510 will give me 80mbps on 2.4

Luxul tech support has not been able to tell me why this is happening..

Just wondering if I am the "only one" with this issue...