Need new Modem for 600Mbs Internet but Xfinity Home Security Wireless Router is weak link

[TEST_ING_123]

I just upgraded to a new 600Mbs plan and would like to achive improved speeds. 
I need a new cable modem becasue my current modem is an old motorola sb6121.
I already have a new fast Wireless router (TPLink).

PROBLEM:
My Xfinity Home Security Wireless router is a NetGear WNR 1000  (seems to be a 10/100 switch) and acts as a pass-through between the old motorola sb6121 and my new wireless router.   My understanding is that the Home Security Wireless router must plug directly into the modem. Please let me know if that is not the case.

Do you have any recomendations to improve my set-up?
 

Thanks!

[TEST_ING_123]

The message on my Home Security Router says: 
"Reconfiguring the Xfinity Home Security Router set-up will result in the loss of your home security broadband and camera, if applicable, connectivity!" 

However, today I was told that I should move the the home security wifi to behind my wirelss router. Will this be a problem? 

[TEST_ING_123]

I moved the home security router to the otherside of my main home wireless router and it's still working. Upgraded to a CM1000 modem and now speeds are over 500Mbs vs 80Mbs before. 

[CCAndrew · Accepted solution]

As long as the security router is getting an internet connection, as you found out, you’re good