Net/http give 404 error when trying to call other route that isn't /

Getting Help
3

first your 404 is file not found, not is route 404.
state 1: your file error.
if / and /example is dir, your request access fild is ../frontend/public/index.html and ../frontend/public/exampme/index.html ,please look net/http/fs.go source code.
/a.js -> ../frontend/public/a.js

[root@localhost tmp]# ls -R bruhurb/ frontend/
bruhurb/:
main.go

frontend/:
public

frontend/public:
example  index.html

frontend/public/example:
index.html
[root@localhost tmp]# cat frontend/public/index.html 
1
[root@localhost tmp]# cat frontend/public/example/index.html 
2
[root@localhost tmp]# cd bruhurb/
[root@localhost bruhurb]# ls
main.go
[root@localhost bruhurb]# go run main.go 
2020/04/08 22:42:53 Listening on http://locahost:8080

state 2: your workdir is tmp dir or workdir is err.
add code fmt.Println(os.Args[0]) show workdir.
if use go run cmd start server, GO use a tmp dir run, workdir is tmp else relative path is invalid, so file is 404.

please use go build and start server in workdir, or add os.Chdir to workdir.