In MX, we have option to call the access-profile in routing-instance, but in EX, the options are limited.
Have you tried to call the routing-instance in access?
labroot@EX42SW# show | compare
[edit]
+ access {
+ radius-server {
+ x.x.x.x {
+ port 1812;
+ secret "$9$ypPrWxbwgJUH24"; ## SECRET-DATA
+ source-address x.x.x.x;
+ routing-instance testA;
+ }
+ }
+ }
+ }