That would be the ideal solution because even if you are out of coverage an antenna can solve your issue. Now you need to be close to the tower and hope the router does not pick a roaming tower. If it does then there is NOTHING you can do about it.
Cellc is filling the gaps in their own 4G coverage with Roaming and there is no way to distinguish from roaming and non roaming towers.
A roaming MTN tower wouldn't register so you'll fall back to a Cell C tower. The issue is with Cell C's map showing both Cell C and MTN coverage and where the Cell C signal is too weak so it can't connect at all.