Depending on how it’s all being wired in, treat the MATT:e box as a consumer unit fed directly from the meter. You might need to get a specific version for that but I know they do one. Otherwise you’d just need to have it inline from your consumer unit.
I had an Ohme fitted recently. The installation company told me prior to the visit that an earthing rod wouldn't be needed because the charger was being installed in the garage. However, I discovered after the installer had finished that he'd decided to put one in anyway (at no extra cost to me). I asked him about it and he said that it was such a simple job (he put it in the drive just under the meter box), he thought he'd do it anyway.