My internet speed is supposed to be 100Mbps down and 10Mbps up, if I connect my PC directly to modem I can measure that, with a degree of error. So far so good. But if I connect PC to modem via router then my speed drops to 10Mbps/10Mbps. I tried Asus RT-AC51U and TP-LINK Archer C20 and both routers drop connection to roughly the same speed even though they should support speeds up to 100mbps. I don't know how to troubleshoot any further, any help would be much appreciated.
EDIT: I used same cable for PC-modem connection as for PC-router connection. I always used same port on modem but tried all 4 ports on router. I can further try other 3 ports on modem and change cable for model-router connection. Will report on that.