Angular 5 with .NET Core 2.1 - Cannot find modules, when it can

Angular 5 with .NET Core 2.1 back-end. This project was started with Visual Studio's combined project template - not the either of the CLI's.

I imported ngx-bootstrap, and confirmed it is loaded correctly in the node_modules folder.

I started the project. Loaded the component. Worked well. Then I started fiddling with the html, and component inputs necessary (followed the directions to get a modal to pop up. https://valor-software.com/ngx-bootstrap/#/modals).

Refreshed the page, and the modal pop-up worked great. The modal appeared and everything was happy.

Closed the solution.

Re-ran the solution with IIS Express.

This is all I get. Nothing in the console except a 404. Nothing in the network tab worth any real info. However, in the "output" box on visual studio, I see the following:

** NG Live Development Server is listening on localhost:4799, open your browser on http://localhost:4799/ **
....

Microsoft.AspNetCore.SpaServices:Error: ERROR in src/app/app.module.ts(7,29): error TS2307: Cannot find module 'ngx-bootstrap/modal'.
src/app/topics/topic.component.ts(6,32): error TS2307: Cannot find module 'ngx-bootstrap/modal'.



Microsoft.AspNetCore.SpaServices:Information: Hash: 79a5ba8829b389c3fa81
Time: 4658ms
chunk {inline} inline.bundle.js (inline) 3.85 kB [entry] [rendered]
chunk {main} main.bundle.js (main) 2.91 kB [initial] [rendered]
chunk {polyfills} polyfills.bundle.js (polyfills) 577 bytes [initial] [rendered]
chunk {scripts} scripts.bundle.js (scripts) 36 kB [initial] [rendered]
chunk {styles} styles.bundle.css (styles) 226 kB [initial] [rendered]
chunk {vendor} vendor.bundle.js (vendor) 852 kB [initial] [rendered]
webpack: Failed to compile.

Failed to load resource: the server responded with a status of 404 () [https://localhost:44390/]
Microsoft.AspNetCore.Hosting.Internal.WebHost:Information: Request finished in 9441.1994ms 404 text/html; charset=utf-8
The program '' has exited with code -1 (0xffffffff).
The program '[2544] dotnet.exe: Program Trace' has exited with code 0 (0x0).
The program '[2544] dotnet.exe' has exited with code -1 (0xffffffff).

How can it suddenly no longer find the module when it just ran flawlessly? Any help?