If the Old Guns quest doesn't start after talking to Preston or after listening to the Minutemen Radio and Ronnie Shaw doesn't appear (after a few days), here is a way to have both her and the Old Guns quest back:
Better be at the Castle but it is not necessary.
1) Install this plugin with any mod manager or (if you are a bear), install it manually in the Data directory of the game. Ronnie should then be at the Castle.
2) In the console, enter 'setstage aa778 50'     That should initialise Old Guns (without notification).
3) Talk to Ronnie and the quest should start.

She should be somewhere (she appears near the main entrance), but if you can't find her, enter: '55ec9.moveto player' to teleport her to you.
If she is naked, enter:
55ec9.equipitem 16892c    (that's the outfit)
55ec9.equipitem 16892e   (the hat)
'inv' shows what she carries.

This plugin just brings Ronnie to the Castle and doesn't solve any other bug like, e.g. when she doesn't sell.
After installing it, everything went normally for me, including Ronnie selling and the radio man staying at his post.
If you have those issues, you could try using Cheat Terminal. In Quests > Minutemen, there is a few options to solve some stuck situations.

Ronnie Shaw and the Old Guns quest were probably bugged in my game because I opened the big mechanised door of the Armoury immediately after taking control of the settlement despite the fact one shouldn't theoretically be able to (I learned later) and I took the 'artillery schematic' and the 'artillery smoke grenades' in the armoury instead of waiting for Ronnie to tell me to...
So, you shouldn't do either of those things as they seem to break that quest more surely than Bethesda could.
But if you did something of the sort or if that bug appeared out of that firm sometimes clumsy programming, this mod should allow you to start the Old Guns quest nonetheless.

The plugin is an ESL with an ESP extension so ESP slots are preserved and only five records are added to the 'ESL space'. The order in the plugin list doesn't matter.

The Quest Wiki page: Old Guns


My other mods
NMCSU Settlement Blueprint - Abernathy Farm
NMCSU Settlement Blueprint - Boston Airport
NMCSU Settlement Blueprint - County Crossing
NMCSU Settlement Blueprint - Covenant
NMCSU Settlement Blueprint - Croup Manor
NMCSU Settlement Blueprint - Finch Farm
NMCSU Settlement Blueprint - Graygarden
NMCSU Settlement Blueprint - Greentop Nursery
NMCSU Settlement Blueprint - Hangman's Alley
NMCSU Settlement Blueprint - Jamaica Plain
NMCSU Settlement Blueprint - Nordhagen Beach
NMCSU Settlement Blueprint - Oberland Station
NMCSU Settlement Blueprint - Spectacle Island
NMCSU Settlement Blueprint - Starlight Drive-in
NMCSU Settlement Blueprint - Sunshine Tidings Co-op
NMCSU Settlement Blueprint - Taffington Boathouse
NMCSU Settlement Blueprint - Tenpines Bluff
NMCSU Settlement Blueprint - Zimonja Outpost

NMCSU Blueprint - Home Plate

NMCSG Plugin - The Castle ready for any Blueprint