I'm afraid you are correct. Most of the FortiWiFi models have only one radio- meaning 2.5G or 5G but not both at the same time.
There is one exception if I recall correctly- a FortiWiFi 50E-2R which does have two radios- but it's a bit of a niche product.
In my view you are always better just getting the Fortigate (in this case the 61F) without WiFi and using a FortiAP. The inbuilt Wireless controller in the Fortigate works well.
The FortiAPs are decent products (arguably not world class). Using FortiAPs provides flexibility for better coverage and AP placement- but obviously makes for higher costs.
If this is home network then I understand cost is an issue- but you won't be disappointment with the Fortigate equipment. It gives great insight into your network and very granular ability to block/ allow traffic. But you do commit yourself to a hefty home IT budget- and quite a bit of learning time.
I guess you purchased a support package? Again you need that to take advantage of web filtering, AV etc.