DFS is short for Dynamic Frequency Selection. It plays a role in 5GHz frequencies that are shared with weather radar.
It is related to 802.11h. DFS support can play a role when using ACS/"survey" in hostapd to find and select free WLAN channels.
Many countries regulate operation on some parts of the 5GHz spectrum - see List_of_WLAN_channels.
Due to fast development, changing hardware, regulatory changes and compliance issues there can be interoperability issues.
OpenWrt uses Open Source drivers which might be abandoned by manufactorer or no longer support some 5GHz operation due to regulatory changes.
Original factory drivers can offer DFS when OpenWrt does not.
There are different DFS schemes: DFS-FCC (USA), DFS-ETSI (Europe), DFS-JP (Japan).
Try to use the non DFS channels if you have old hardware/clients.