i fixed with 2.1

script.on_event(defines.events.on_tick, function(event)
if event.tick % 2 == 0 then
return
end

local playerDictionary = game.connected_players

--  set if diagonal entrances to underground belts is allowed (Default: On)
local diff
local allowedDiff;
if settings.global["tunneler-diagonal-checking"].value then
    allowedDiff = 2
else
    allowedDiff = 0
end

--  guard against nils, then if the player has belt immunity equipment and has the setting turned on skip all the tunneler logic
for var, player in pairs(playerDictionary) do
    if player.character ~= nil and settings.global["tunneler-equipment-immunity"].value
        and player.character.grid ~= nil
        and player.character.grid.find("belt-immunity-equipment")
    then

        if settings.global["tunneler-debug"].value then
            player.print("Player collided with belt but has belt immunity.", { color = { 225, 0, 0 } })
        end
        goto skipPoint
    end

    local walkState = player.walking_state

    -- if player is walking, check for underground belts within the set radius
    if walkState.walking then
        local localEntities = player.surface.find_entities_filtered {
            position = player.position,
            radius = settings.global["tunneler-radius"].value,
            type = "underground-belt",
        }
        for var, belt in pairs(localEntities) do
            --  for each underground belt calculate if the player is facing it, if it's an input belt, and if so
            --  teleport to other end of the belt
            diff = math.abs(walkState.direction - belt.direction)
            diff = math.min(diff, 16 - diff)
            if belt.belt_to_ground_type == "input" and diff <= allowedDiff then
                if settings.global["tunneler-debug"].value then player.print(
                    "Player collided with " .. belt.name .. "!", { color = { 225, 225, 0 } }) end
                -- `neighbours` was removed in Factorio 2.1. This property is
                -- the underground belt endpoint, or nil when it is unconnected.
                local outputBelt = belt.underground_belt_neighbour
                if outputBelt ~= nil then
                    player.teleport(outputBelt.position, player.surface)
                    player.play_sound { path = "whoosh", volume_modifier = settings.global["tunneler-volume"].value }
                    if settings.global["tunneler-debug"].value then player.print(
                        "Teleported player to " .. outputBelt.position.x .. ", " .. outputBelt.position.y .. ".",
                        { color = { 225, 225, 0 } }) end
                else
                    if settings.global["tunneler-debug"].value then
                        player.print(
                            "Couldn't teleport due to unconnected belt!", { color = { 225, 0, 0 } })
                    end
                end
            end
        end
    end
    ::skipPoint::
end

end)

Thanks for letting me know, I never tested with 2.1 so thats on me for letting it run 2.1 in the first place.

Have fixed and uploaded a dedicated 2.1 version of the mod