Typegoose and NestJS: Property 'save' does not exist on type

I am using typegoose and nestjs for my backend-server. I already have a function in my pages.service.ts file to get a single page by ID called getPageById(). When i try to call this function from another inside my pages.services.ts file, i get the following error by typescript:

Property 'save' does not exist on type 'page'

my page.model.ts file looks like this

import { DocumentType, modelOptions, prop, Severity } from "@typegoose/typegoose";
import { Content } from "./models/content.model";

@modelOptions({
    schemaOptions: {
        timestamps: true,
        toJSON: {
            transform: (doc: DocumentType<Page>, ret) => {
                delete ret.__v;
                ret.id = ret._id;
                delete ret._id;
            }
        }
    },
    options: {
        allowMixed: Severity.ALLOW
    }
})
export class Page {
    @prop({required: true})
    title: string;

    @prop({required: true})
    description: string;

    @prop({required: true})
    content: Content;

    @prop()
    createdAt?: Date;

    @prop()
    updatedAt?: Date;

    @prop()
    category: string;
}

and my pages.service.ts file looks like this

import { Injectable, NotFoundException } from '@nestjs/common';
import { ReturnModelType } from '@typegoose/typegoose';
import { InjectModel } from 'nestjs-typegoose';
import { createPageDto } from './dto/create-page.dto';
import { Page } from './page.entity';

@Injectable()
export class PagesService {
    constructor(
        @InjectModel(Page)
        private readonly pageModel: ReturnModelType<typeof Page>
    ) {}

    async getPageById(id: string): Promise<Page> {
        let page;
        try {
            page = await this.pageModel.findById(id);
        } catch (error) {
            throw new NotFoundException(`Page could not be found`);
        }
        if (!page) {
            throw new NotFoundException(`Page could not bet found`);
        }
        return page;
    }

    async updatePageCategory(id: string, category: string): Promise<Page> {
        const page = await this.getPageById(id);
        page.category = category;
        page.save() // i get the error here
        return page;
    }
}

What do i need to get this working?

Update

I could fix the bug. I changed the return type to Promise<DocumentType<Page>> like this

async getPageById(id: string): Promise<DocumentType<Page>> {
    let page;
    try {
        page = await this.pageModel.findById(id);
    } catch (error) {
        throw new NotFoundException(`Page could not be found`);
    }
    if (!page) {
        throw new NotFoundException(`Page could not bet found`);
    }
    return page;
}

But is this the best way to solve this?