@MAXiMUSMX-TH
Go to this menu:

scroll down to the bottom of that page and observe the negotiated speed of the WAN (Internet) port:

If the negotiated speed is 1000mbps full duplex then check this setting:

If the setting is on Auto Negotiation then probably the port of the equipment before the AX11000 is 1000mbnps capable and that's why you can't negotiate more bandwidth.
To achieve 2.5gbps you have to use an CAT6/6A ethernet cable and the port of the equipment to which the AX11000's WAN port is connected, should be 2.5gbps capable.