SR808ac - SmartRG compatability

I have a new SmartRG SR808ac combo modem/router. Looking to buy a range extender but on a budget. Would the Netgear Ex6120 be a good choice. Have a 4 story narrow townhouse with SmartRG in back office on second floor. Weaker signal on first story front room. I'm assumming these are compatable with the new combo router/modems. Thank you.