I was having an issue with Big Sur on both iMac and MacBook and Synology NAS accessing shares when coming out of sleep. HTTP access worked ok. Posted here and eventually with great help got it going better. Not 100% (other factors maybe involved … dunno) with a drop or two each month–fixed by re-boot of Mac. Sometimes I power off/on the router to fix.
See Reconnecting to Synology NAS After "sleep" seems impossible : Fixed - #14 by csf111