When I flashed OpenWrt for the first time I achieved my ISP full speed on Ethernet and WiFi "2.4". However after two weeks speed dropped suddenly from 256Mbs to 90Mbs on Ethernet and from 256Mbs to 90Mbs over WiFi.
Router is : Linksys 1900ACS V2
Connection is: PPPOE
perhaps your ISP suddenly capped you ?
Check your WAN link speed