It is my router :
When the router boot for a long time about 24 h, the wlan1(2.4G) can't connect but wlan0 (5G) work well.
I recently saw that Mediatek MT76 had some problems... I think that some of them were recently resolved, follow up here: https://github.com/openwrt/mt76 and feel free to try recent trunk/snapshots bin files using sysupgrade.
you have there (screenshot) release r4433, current one today is r4527.
post your Wireless Config uci show wireless with Kernel log and System Log too dmesg and logread , those should be useful.