Change the game to match the players, not the other way around.
In your scenario, with players avoiding combat whenever possible, it's the DM's responsibility to make successes feel like successes. There may be a mismatch between player types, with the players wanting to avoid combat, and you wanting to create combat, but that may mean that another DMing playstyle needs to be used for everyone to get the most out of the game.
Assuming that you're willing to adapt the game to make it feel more rewarding for your players, there's a few things you want to note:
Modules are designed to be fluid, to allow the DM to modify as needed. Maybe there wasn't a secret entrance there, until your players started looking for one. Maybe there wasn't a trap in the floor, until the Rogue started checking for traps constantly. Meet your players' expectations by creating things that they're looking for. In turn, they'll create the world for you while you just give the people what they want.
Experience and other rewards can be obtained through a bunch of different methods other than combat, but it's up to the DM to decide upon the best method. Some DMs use a per-session-attendance level scheme, where the number of sessions you need to attend to level up is equal to your current level (so it'll take you 5 sessions to go from level 5 to level 6), which is good for campaigns with long RP times or to incentivize attendance. Other times, simply finishing a particular sequence in the module is enough to earn a level (usually stated in the module itself when this happens), which is good for balance. Other DMs reward EXP for not slaying an obstacle/enemy, but simply pacifying it, which is good for rewarding unorthodox solutions. Choose the method that works best for the table.
Forcing combat is something that's within your power (via ambushes, traps, imprisoning, etc), but make sure that the game is designed around players having fun. There are not many undead in Lost Mines of Phandelver, but if the players decided to make a party of holy Paladins and Clerics, I'd want them to stand out and feel rewarded for their decisions, so I might swap out some generic enemies for some generic undead. Consider having enemies adapted to the player's usual interactions, or providing clues as to how the players' expertise can deal with these scenarios.
But what should I do about right now?
For it being this late into the module, the good solutions are limited, so it's important to attempt to make good habits going forward so that this issue doesn't pop up in the future.
Applying a short-term Band-Aid, like making the dungeon easier, is an option, but it might create problems in the event that the players can't pacify an obstacle, or they decide to play after the module is finished. To avoid any future issues with the module and any events afterwards, the focus should be on leveling the players immediately rather than constantly trying to scale the game back.
With the style of play of your players, there are two methods I'd recommend to increase their level to the appropriate threshold:
Level them immediately, stating that you've decided to change how Experience was rewarded in your campaign, and that their current Experience values are adjusted for these changes and will stay at this rate going forward (putting them at whatever experience method you think is appropriate). If you want to avoid just handing out free levels without it feeling justified, try and calculate their current experience values by determining how you feel they should earn experience and running some rough calculations by skimming through what the players went through. If you're shifting to a benchmark system, make everyone's level the same up to the expected amount. If you're now rewarding EXP for pacifying creatures, just skim over what combats they avoided and tally up their new EXP. Once you've done that, explain to your players what the changes are and how their new levels reflect that.
Railroading them onto a detour, whether that be by aiding an allied army push back a band of marauding goblins, or by defending a town from monsters. Railroading, like a scalpel, is a tool best used with precision and becomes scary with excessive use, so while Railroading is generally frowned upon as a default method to DM, this is a perfect scenario for it. Use this time to utilize a sidequest your players managed to avoid, modifying it to accommodate their current situation, and tie in something that effectively requires them to participate in it, whether that's an important NPC asking a favor or an item they require for their mission. Perhaps them ignoring a sidequest they could have taken care of now evolved into a greater threat that they cannot ignore.
With any solution you choose, make sure that the players are recognized for what they're trying to get out of the game, and try to adapt the game to that, even if that means that the enemies are more easily manipulated or that they're more easily thwarted by items hidden in the dungeon that the players can find.
Because of the fact that leveling them immediately lets your players feel an immediate reward for their uphill slog and also sets the expectation that they'll be rewarded for their playstyle in the future on both sides of the table, I think that it is the best approach for your campaign. Just make sure that both sides recognize that it's a reward (compensation) rather than an award (gift).
