Have an issue with a slow wired connection - looking for some ideas as to how to identify issue and rectify.  
 

setup is 

Archer VR2100 router

ISP is Hey Broadband and have 1gb FTTP - have their modem ( Adtran SDX611) in bridge mode so I can use my own router

Own router is in the - wireless router mode

QOS is disabled

laptop connected via Ethernet cable directly to ISP modem gives expected upload / download speeds - 900mb ( used both cat5 and cat6 cables )

when connecting laptop to LAN port on router, speeds drops by more than half  to circa 300mb - same issue using cat5 or cat6 cables (cable types not mixed)

firmware is current 

wireless speeds are less than the decreased wired connection though laptop connection speed to router shows upwards of 700mb  - but suspect when I fix the wired issue then wireless will improve 

Any suggestions appreciated.