el captain can't see network device shares on 2.4Ghz Wifi

Hi, I have a home network with a wifi router with both a 5Ghz and 2.4Ghz signal. I usually use the 2.4Ghz as the signal is usually stronger in my home. Also on my network is a Synology NAS which has a TimeMachine service. All has worked well for quite awhile until last week, when there were updates to my MacBookPro and the Synology NAS.

I now find that the Synology Diskstation NAS, and other devices on the network do not show up in FInder under Shared when the Macbook connects to the 2.4Ghz network, but strangely they do show up if the MacBook connects to the 5Ghz network; if I then switch back to the 2.4Ghz the devices are there for awhile before disappearing.

I can use the FInder's Go->Connect To Server... menu item to connect to a share on a device by explicitly entering the path to the share, even when on the 2.4Ghz. However, the NAS device name doesn't turn up in the "Select DIsk" option of TimeMachine (when connected via 2.4Ghz) as it used to.

I've rebooted the Mac, NAS and router but that hasn't changed things. I've disabled firewalls but still the issue persists.

Strikes me as a really weird issue. Has anybody seen anything similar?

Cheers,
Paul