If you are using system folders for mouting there is no way you can control the permissions.
You can use the utility script mentioned in the troubleshooting topic
create a separate structure
In the topic linked previously it is suggested you create a separate structure e.g.
mkdir -p /a/smb/nmshare
The you can control the local permissions for the mountpoint
sudo chmod ugo+rwx /a/smb/nmshare
mount the share
Then mount the share in the folder using a mount unit
First create a folder to hold your credentials for the server
sudo mkdir -p /etc/credentials
Secure the folde to root acess only
sudo chmod 700 /etc/credentials
Create a file in the folder to hold the credentials - for convenience name the file as router-share-user
sudo touch /etc/credentials/router-share-$USER
Edit the file using your favorite editor
Insert the following lines a total of 3 (three) lines - content in that order - and save the file. The following is an example based on your comments
Create a file with a name matching the mount point
sudo touch /etc/systemd/system/a-smb-nmshare.mount
Edit the mount unit using your favorite editor
Insert the following content - replace $USER with the actual username e.g. admin
You may not need the part specifying the protocol version - it is there because routers with old firmware uses the insecure SMB1 which was renamed to NT1 to provide backwards compatibility.
TEST the share by starting the mount unit.
sudo systemctl start a-smb-nmshare.mount
Navigate to the folder /a/smb/nmshare using your filemanager. You should now see the content of the share - if not check your credentials. Test if you can create a new file - then delete the file.
IF this is a workstation - always connected to the router - continue to enable the unit at boot - otherwise continue in next section
sudo systemctl enable a-smb-nmshare.mount
IF this is a laptop which may or may not be conected to your home network stop the mount unit
sudo systemctl stop a-smb-nmshare.mount
do not enable the mount unit but create an additional automount unit
sudo touch /etc/systemd/system/a-smb-nmshare.automount
Edit the file and insert the following content
Then enable and start the unit
sudo systemctl enable --now a-smb-nmshare.automount
Navigate to the folder /a/smb/nmshare and it will mount in the background and unmount after an idle time of 10s.