CMP doesn't recognise variable as object instance

[AntoninoIemmolo]

I'm trying to use cmp for some object oriented programming in LUA. I'm using lsp as language server and these are my pretty basic configuration:
LSP config.

local lsp = require("lsp-zero")
require("mason").setup()
require("mason-lspconfig").setup({
    ensure_installed={"lua_ls","clangd"}
})
require("lspconfig").lua_ls.setup{
  capabilities = capabilities,
    settings={
        Lua={
            workspace={
                maxpreload=200,
                preloadFileSize=75000,
            }
        }
    }
}
require("lspconfig").clangd.setup{}
lsp.preset('racommended')
lsp.setup()

cmp config:

-- Set up nvim-cmp.
  local cmp = require'cmp'

  cmp.setup({
    snippet = {
      -- REQUIRED - you must specify a snippet engine
      expand = function(args)
        require('luasnip').lsp_expand(args.body) -- For `luasnip` users.
      end,
    },
    window = {
      completion = cmp.config.window.bordered(),
      ---documentation = cmp.config.window.bordered(),

    },
    mapping = cmp.mapping.preset.insert({
    ["<C-k>"] = cmp.mapping.select_prev_item(), -- previous suggestion
    ["<C-j>"] = cmp.mapping.select_next_item(), -- next suggestion
    ["<C-b>"] = cmp.mapping.scroll_docs(-4),
    ["<C-f>"] = cmp.mapping.scroll_docs(4),
    --["<Tab>"] = cmp.mapping.complete(), -- show completion suggestions
    ["<C-e>"] = cmp.mapping.abort(), -- close completion window
    ["<CR>"] = cmp.mapping.confirm({ select = false }),
    }),
    sources = cmp.config.sources({
      { name = 'nvim_lsp' },
      { name = 'luasnip' },
      { name = "path" },
      { name = 'buffer' },

    }),
    formatting = {
        format = function(entry, vim_item)
            vim_item.abbr = string.sub(vim_item.abbr, 1, 40)
            return vim_item
        end
    }
  })

  -- Set configuration for specific filetype.
  cmp.setup.filetype('gitcommit', {
    sources = cmp.config.sources({
      { name = 'git' }, -- You can specify the `git` source if [you were installed it](https://github.com/petertriho/cmp-git).
    }, {
      { name = 'buffer' },
    })
  })

  -- Use buffer source for `/` and `?` (if you enabled `native_menu`, this won't work anymore).
  cmp.setup.cmdline({ '/', '?' }, {
    mapping = cmp.mapping.preset.cmdline(),
    sources = {
      { name = 'buffer' }
    }
  })

  -- Use cmdline & path source for ':' (if you enabled `native_menu`, this won't work anymore).
  cmp.setup.cmdline(':', {
    mapping = cmp.mapping.preset.cmdline(),
    sources = cmp.config.sources({
      { name = 'path' }
    }, {
      { name = 'cmdline' }
    })
  })

  -- Set up lspconfig.
  capabilities = require('cmp_nvim_lsp').default_capabilities()

That said, when I type something like "AIRBASE:" (assuming AIRBASE is an object declared in an other Lua file) I get the all the method and function associated with AIRBASE as shown in the first picture below.
But, and here comes the problem, when I type something a little bit fancier like

local foo = AIRBASE:FindByName("fooName")
foo:

cmp doesn't give me the method anymore even if FindByName() is a function which is supposed to return and AIRBASE istance.
Note that if I write:

local foo = AIRBASE
foo:

still gives me al the method associated with AIRBASE